Graphene oxide, positive electrode for nonaqueous secondary battery using graphene oxide, method of manufacturing positive electrode for nonaqueous secondary battery, nonaqueous secondary battery, and electronic device

ABSTRACT

A graphene oxide used as a raw material of a conductive additive for forming an active material layer with high electron conductivity with a small amount of a conductive additive is provided. A positive electrode for a nonaqueous secondary battery using the graphene oxide as a conductive additive is provided. The graphene oxide is used as a raw material of a conductive additive in a positive electrode for a nonaqueous secondary battery and, in the graphene oxide, the atomic ratio of oxygen to carbon is greater than or equal to 0.405.

TECHNICAL FIELD

The present invention relates to a graphene oxide, a positive electrodefor a nonaqueous secondary battery which uses the graphene oxide, amethod of manufacturing the positive electrode for a nonaqueoussecondary battery, a nonaqueous secondary battery, and electronicdeices.

BACKGROUND ART

With the recent rapid spread of portable electronic devices such ascellular phones, smartphones, electronic books, and portable gamemachines, secondary batteries for drive power supply have beenincreasingly required to be smaller and to have higher capacity.Nonaqueous secondary batteries typified by lithium secondary batteries,which have advantages such as high energy density and high capacity,have been widely used as secondary batteries used for portableelectronic devices.

A lithium secondary battery, which is one of nonaqueous secondarybatteries and widely used due to its high energy density, includes apositive electrode including an active material such as lithium cobaltoxide (LiCoO₂) or lithium iron phosphate (LiFePO₄), a negative electrodeformed of a carbon material such as graphite which is capable ofocclusion and release of lithium ions, a nonaqueous electrolyte in whichan electrolyte formed of a lithium salt such as LiBF₄ or LiPF₆, and thelike is dispersed in an organic solvent such as ethylene carbonate ordiethyl carbonate, and the like. A lithium secondary battery are chargedand discharged in such a way that lithium ions in the secondary batteryare transferred between the positive electrode and the negativeelectrode through the nonaqueous electrolyte and intercalated into ordeintercalated from the active materials of the positive electrode andthe negative electrode.

Into the positive electrode or the negative electrode, a binding agent(also referred to as a binder) is mixed in order that active materialscan be bound or an active material and a current collector can be bound.Since the binding agent is generally an organic high molecular compoundsuch as polyvinylidene fluoride (PVDF) which has an insulating property,the electron conductivity of the binding agent is extremely low.Therefore, as the ratio of the mixed binding agent to the activematerial is increased, the amount of the active material in theelectrode is relatively decreased, resulting in the lower dischargecapacity of the secondary battery.

Hence, by mixture of a conductive additive such as acetylene black (AB)or a graphite particle, the electron conductivity between activematerials or between an active material and a current collector can beimproved. Thus, a positive electrode active material with high electronconductivity can be provided (see Patent Document 1).

REFERENCE

-   Patent Document 1: Japanese Published Patent Application No.    2002-110162

DISCLOSURE OF INVENTION

However, because acetylene black used as a conductive additive is ahigh-volume particle with an average particle diameter of several tensof nanometers to several hundreds of nanometers, contact betweenacetylene black and an active material hardly becomes surface contactand tends to be point contact. Consequently, contact resistance betweenthe active material and the conductive additive is high. Further, if theamount of the conductive additive is increased so as to increase contactpoints between the active material and the conductive additive, theproportion of the amount of the active material in the electrodedecreases, resulting in the lower discharge capacity of the battery.

In the case where graphite particles are used as a conductive additive,natural graphite is generally used in consideration of cost. In thiscase, iron, lead, copper, or the like contained as impurities in agraphite particle reacts with the active material or the currentcollector, which might reduce the potential or capacity of the battery.

Further, as particles of the active material become minuter, cohesionbetween the particles becomes stronger, which makes uniform dispersionin the binding agent or the conductive additive difficult. Consequently,a portion where active material particles are aggregated and denselypresent and a portion where active material particles are not aggregatedand thinly present are locally generated. In the portion where activematerial particles are aggregated and to which the conductive additiveis not mixed, the active material particles do not contribute toformation of the discharge capacity of the battery.

Therefore, in view of the foregoing problems, an object of oneembodiment of the present invention is to provide a graphene oxide whichis a raw material of a conductive additive used for an active materiallayer which achieves high electron conductivity with a small amount of aconductive additive. Another object is to provide, with a small amountof a conductive additive, a positive electrode for a nonaqueoussecondary battery which is highly filled and includes a high-densitypositive electrode active material layer. Another object is to provide,using the positive electrode for a nonaqueous secondary battery, anonaqueous secondary battery having high capacity per electrode volume.

A positive electrode for a nonaqueous secondary battery in accordancewith one embodiment of the present invention includes a graphene as aconductive additive included in a positive electrode active materiallayer.

A graphene is a carbon material having a crystal structure in whichhexagonal skeletons of carbon are spread in a planar form and is oneatomic plane extracted from graphite crystals. Due to its electrical,mechanical, or chemical characteristics which are surprisinglyexcellent, the graphene has been expected to be applied to a variety offields of, for example, field-effect transistors with high mobility,highly sensitive sensors, highly-efficient solar cells, andnext-generation transparent conductive films and has attracted a greatdeal of attention.

In this specification, the term graphene includes a single-layergraphene and multilayer graphenes including two to hundred layers. Thesingle-layer graphene refers to a sheet of one atomic layer of carbonmolecules having π bonds. The graphene oxide refers to a compound formedby oxidation of such a graphene. Note that when a graphene oxide isreduced to form a graphene, oxygen contained in the graphene oxide isnot entirely released and part of oxygen remains in the graphene. Whenthe graphene contains oxygen, the proportion of oxygen is greater thanor equal to 2 atomic % and less than or equal to 20 atomic %, preferablygreater than or equal to 3 atomic % and less than or equal to 15 atomic%.

In the case where the graphene is multilayer graphenes including thegraphene obtained by reducing the graphene oxide, the interlayerdistance between the graphenes is greater than or equal to 0.34 nm andless than or equal to 0.5 nm, preferably greater than or equal to 0.38nm and less than or equal to 0.42 nm, more preferably greater than orequal to 0.39 nm and less than or equal to 0.41 nm. In general graphite,the interlayer distance between single-layer graphenes is 0.34 nm. Sincethe interlayer distance between the graphenes used for the secondarybattery of one embodiment of the present invention is longer than thatin general graphite, carrier ions can easily transfer between thegraphenes in multilayer graphenes.

In a positive electrode for a nonaqueous secondary battery in accordancewith one embodiment of the present invention, graphenes are overlappedwith each other in a positive electrode active material layer anddispersed so as to be in contact with a plurality of positive electrodeactive material particles. In other words, a network for electronconduction is formed by the graphenes in a positive electrode activematerial layer. This maintains bonds between the plurality of positiveelectrode active material particles, which enables a positive electrodeactive material layer with high electron conductivity to be formed.

A positive electrode active material layer to which a graphene is addedas a conductive additive can be manufactured by the following method.First, after the graphene is dispersed into a dispersion medium (alsoreferred to as a solvent), a positive electrode active material is addedthereto and a mixture is obtained by mixing. A binding agent (alsoreferred to as a binder) is added to this mixture and mixing isperformed, so that a positive electrode paste is formed. Lastly, afterthe positive electrode paste is applied on a positive electrode currentcollector, the dispersion medium is volatilized. Thus, the positiveelectrode active material layer to which the graphene is added as aconductive additive is manufactured.

In order that a network for electron conduction can be formed in apositive electrode active material layer with use of the graphene as aconductive additive, the graphene needs to be uniformly dispersed in thedispersion medium. Dispersibility in a dispersion medium directlydepends on the dispersibility of the graphene in a positive electrodeactive material layer. When the dispersibility of the graphene is low,the graphene is aggregated in the positive electrode active materiallayer and localized, which prevents formation of the network. Thus thedispersibility of the graphene used as a conductive additive in adispersion medium is an extremely important factor in the improvement ofthe electron conductivity of the positive electrode active materiallayer.

By examining a positive electrode active material layer formed in such away that a graphene as a conductive additive was put in a dispersionmedium together with an active material and a binding agent, the presentinventors found that dispersibility was insufficient and a network forelectron conduction was not formed in the positive electrode activematerial layer. The inventors found the same results by examining apositive electrode active material layer formed in such a way that,instead of a graphene, a graphene formed by reduction of a grapheneoxide (hereinafter, referred to as an RGO (an abbreviation of reducedgraphene oxide)) was put as a conductive additive in a dispersionmedium.

In contrast, the present inventors have found that excellent electronconductivity is achieved by formation of a network for electronconduction in a positive electrode active material layer obtained insuch a way that, after a graphene oxide (also referred to as a GO) as aconductive additive is put in a dispersion medium together with anactive material and a binding agent to form a positive electrode paste,the dispersed graphene oxide is reduced by heat treatment to form agraphene.

Thus, while dispersibility is low in a positive electrode activematerial layer in which a graphene or a RGO is dispersed as a rawmaterial of a conductive additive, high dispersibility is achieved witha graphene formed by reduction performed after a graphene oxide is addedto form a positive electrode paste.

Such a difference in the dispersibility in an active material layerbetween the graphene or RGO and the graphene formed by reductionperformed after a positive electrode paste including a graphene oxide isformed can be explained below as a difference in the dispersibility in adispersion medium.

FIG. 1A illustrates a structural formula of NMP (also referred to asN-methylpyrrolidone, 1-methyl-2-pyrrolidone, N-methyl-2-pyrrolidone, orthe like), which is a typical dispersion medium. An NMP 100 is acompound having a five-membered ring structure and is one of polarsolvents. As illustrated in FIG. 1A, in the NMP, oxygen is electricallynegatively biased and carbon forming a double bond with the oxygen iselectrically positively biased. A graphene, an RGO, or a graphene oxideis added to a diluent solvent having such a polarity.

The graphene is a crystal structure body of carbon in which hexagonalskeletons are spread in a planar form as already described, and does notsubstantially include a functional group in the structure body. Further,the RGO is formed by reduction of functional groups originally includedin the RGO by heat treatment, and the proportion of functional groups inthe structure body is as low as about 10 wt %. Consequently, asillustrated in FIG. 1B, a surface of a graphene or RGO 101 does not havepolarity and therefore has hydrophobicity. Therefore it is consideredthat, while interaction between the NMP 100 which is a dispersion mediumand the graphene or RGO 101 is extremely weak, interaction occursbetween the graphenes or RGOs 101 to cause aggregation of the graphenesor RGOs 101 (see FIG. 1C).

A graphene oxide 102 is a polar substance having a functional group suchas an epoxy group, a carbonyl group, a carboxyl group, or a hydroxylgroup. Oxygen in the functional group in the graphene oxide 102 isnegatively charged; hence, graphene oxides hardly aggregate in a polarsolvent but strongly interact with the NMP 100 which is a polar solvent(see FIG. 2A). Thus, as illustrated in FIG. 2B, the functional groupsuch as an epoxy group included in the graphene oxide 102 interacts witha polar solvent, which inhibits aggregation among graphene oxides;consequently, the graphene oxide 102 is considered to be uniformlydispersed in a dispersion medium (see FIG. 2B).

In view of the foregoing, in order that a network with high electronconductivity be formed in a positive electrode active material layer byusing the graphene as a conductive additive, use of the graphene oxidewith high dispersibility in a dispersion medium in manufacture of apositive electrode paste is very effective. The dispersibility of thegraphene oxide in a dispersion medium is considered to depend on thequantity of functional groups having oxygen such as an epoxy group(i.e., the degree of oxidation of the graphene oxide).

One embodiment of the present invention is a graphene oxide used as araw material of a conductive additive in a positive electrode for anonaqueous secondary battery. In the graphene oxide, the atomic ratio ofoxygen to carbon is greater than or equal to 0.405.

Here, the atomic ratio of oxygen to carbon is an indicator of the degreeof oxidation and represents the atomic of oxygen which is a constituentelement of the graphene oxide as a proportion with respect to the atomicof carbon which is a constituent element of the graphene oxide. Notethat the atomic of elements included in the graphene oxide can bemeasured by X-ray photoelectron spectroscopy (XPS), for example.

The atomic ratio of oxygen to carbon in the graphene oxide which isgreater than or equal to 0.405 means that the graphene oxide is a polarsubstance in which functional groups such as an epoxy group, a carbonylgroup, a carboxyl group, or a hydroxyl group are sufficiently bonded tothe graphene oxide for the high dispersibility of the graphene oxide ina polar solvent.

The graphene oxide in which the atomic ratio of oxygen to carbon isgreater than or equal to 0.405 is dispersed in a dispersion mediumtogether with a positive electrode active material and a binding agent,the mixture is mixed, the mixture is applied on a positive electrodecurrent collector, and heating are performed. Thus, a positive electrodefor a nonaqueous secondary battery which includes a graphene with highdispersibility and a network for electron conduction can be formed.

The length of one side of the graphene oxide is preferably greater thanor equal to 50 nm and less than or equal to 100 μm, more preferablygreater than or equal to 800 nm and less than or equal to 20 μm.

Another embodiment of the present invention is a positive electrode fora nonaqueous secondary battery which includes a positive electrodeactive material layer including a plurality of positive electrode activematerial particles, a conductive additive including a plurality ofgraphenes, and a binding agent over a positive electrode currentcollector. Each of the graphenes is larger than an average particlediameter of each of the positive electrode active material particles.Each of the graphenes is dispersed in the positive electrode activematerial layer such that the graphene makes surface contact with one ormore graphenes adjacent to the graphene. The graphenes make surfacecontact in such a way as to wrap part of surfaces of the positiveelectrode active material particles.

As already described, the graphene oxides are structure bodies havingfunctional groups including oxygen and therefore do not aggregate andare uniformly dispersed in a polar solvent such as NMP. The dispersedgraphene oxides uniformly mix with the plurality of positive electrodeactive material particles. Thus, the graphenes, which are formed fromthe graphene oxide by volatilization of the dispersion medium andreduction treatment of the graphene oxide, are dispersed in the positiveelectrode active material layer such that the graphenes make surfacecontact with each other. Since the graphene has a sheet-like shape andpartial surface contact between the graphenes achieves electricalconnection, a network for electron conduction is considered to be formedwhen some graphenes are viewed as one set. Further, the surface contactbetween the graphenes can keep contact resistance low, which leads tothe formation of the network with high electron conductivity.

Further, since the graphene is a sheet whose side has a length greaterthan or equal to 50 nm and less than or equal to 100 μm, preferablygreater than or equal to 800 nm and less than or equal to 20 μm, whichis larger than an average particle diameter of the positive electrodeactive material particles, the graphene in the form of a sheet can beconnected to the plurality of positive electrode active materialparticles. In particular, since the graphene has a sheet-like shape,surface contact can be made in such a way as to wrap the surfaces of thepositive electrode active material particles. Accordingly, without anincrease in the amount of conductive additive, contact resistancebetween the positive electrode active material particles and thegraphenes can be reduced.

Note that as the positive electrode active material particles, amaterial capable of inserting and extracting of carrier ions, such aslithium iron phosphate, can be used.

Another embodiment of the present invention is a positive electrode fora nonaqueous secondary battery which includes a positive electrodeactive material layer including a plurality of positive electrode activematerial particles, a conductive additive including a plurality ofgraphenes, and a binding agent over a positive electrode currentcollector. As bonding states of carbon included in the positiveelectrode active material layer, the proportion of a C═C bond is greaterthan or equal to 35% and the proportion of a C—O bond is greater than orequal to 5% and less than or equal to 20%.

Another embodiment of the present invention is a method of manufacturinga positive electrode for a nonaqueous secondary battery, which includesthe steps of: dispersing a graphene oxide in which the atomic ratio ofoxygen to carbon is greater than or equal to 0.405 into a dispersionmedium; adding a positive electrode active material to the dispersionmedium into which the graphene oxide is dispersed and performing mixingto form a mixture; adding a binding agent to the mixture and performingmixing to form a positive electrode paste; applying the positiveelectrode paste on a positive electrode current collector; and reducingthe graphene oxide after or at the same time when the dispersion mediumincluded in the applied positive electrode paste is volatilized, wherebya positive electrode active material layer including the graphene isformed over the positive electrode current collector.

The length of one side of each of the graphene oxide and the graphene ispreferably greater than or equal to 50 nm and less than or equal to 100μm, more preferably greater than or equal to 800 nm and less than orequal to 20 μm.

In the above manufacturing method, the positive electrode paste is driedunder a reducing atmosphere or reduced pressure. This enables thedispersion medium included in the positive electrode paste to bevolatilized and the graphene oxide included in the positive electrodepaste to be reduced.

In the above manufacturing method, by further addition of a dispersionmedium at the time when the binding agent is added to the mixture andmixing is performed, the viscosity of the positive electrode paste canbe adjusted.

The positive electrode active material is added to the dispersion mediumin which the graphene oxide with an atomic ratio of oxygen to carbongreater than or equal to 0.405 is dispersed. The resulting substance ismixed, so that the positive electrode active material layer with highdispersibility of the graphene is formed. The graphene oxide can beincluded at least at 2 wt % with respect to the total weight of thepositive electrode paste which is a mixture of the positive electrodeactive material, the conductive additive, and the binding agent.Further, the graphene obtained after the positive electrode paste isapplied on the current collector and reduction is performed can beincluded at least at 1 wt % with respect to the total weight of thepositive electrode active material layer. This is because the weight ofthe graphene is reduced by almost half due to the reduction of thegraphene oxide.

Specifically, it is preferable that, in the state of the positiveelectrode paste, the graphene oxide be added at greater than or equal to2 wt % and less than or equal to 10 wt %, the positive electrode activematerial be added at greater than or equal to 85 wt % and less than orequal to 93 wt %, and the binding agent be added at greater than orequal to 1 wt % and less than or equal to 5 wt %, with respect to thetotal weight of the positive electrode paste. Further, it is preferablethat, in the state of the positive electrode active material layerobtained by applying the positive electrode paste on the currentcollector and reducing the graphene oxide, the graphene be added atgreater than or equal to 1 wt % and less than or equal to 5 wt %, thepositive electrode active material be added at greater than or equal to90 wt % and less than or equal to 94 wt %, and the binding agent beadded at greater than or equal to 1 wt % and less than or equal to 5 wt%, with respect to the total weight of the positive electrode activematerial layer.

After the positive electrode paste is applied on the positive electrodecurrent collector, oxygen is released from the graphene oxide by dryingunder a reducing atmosphere or reduced pressure, so that the positiveelectrode active material layer including the graphene can be formed.Note that oxygen included in the graphene oxide is not entirely releasedand may partly remain in the graphene.

When the graphene includes oxygen, the proportion of oxygen is greaterthan or equal to 2 atomic % and less than or equal to 20 atomic %,preferably greater than or equal to 3 atomic % and less than or equal to15 atomic %. As the proportion of oxygen is lower, the conductivity ofthe graphene can be higher, so that a network with high electronconductivity can be formed. As the proportion of oxygen is higher, moreopenings serving as paths of ions can be formed in the graphene.

By using the positive electrode formed in the above-described manner, anegative electrode, an electrolyte solution, and a separator, anonaqueous secondary battery can be manufactured.

A graphene oxide which is a raw material of a conductive additive usedfor an active material layer which achieves high electron conductivitycan be provided with a small amount of a conductive additive.

By using the graphene oxide as a raw material of a conductive additive,a positive electrode for a nonaqueous secondary battery including apositive electrode active material layer which can achieve high electronconductivity can be provided with a small amount of a conductiveadditive. A high-density positive electrode for a nonaqueous secondarybattery which includes a positive electrode active material layer whichis highly filled can be provided with a small amount of a conductiveadditive.

By using the positive electrode for a nonaqueous secondary battery, anonaqueous secondary battery having high capacity per electrode volumecan be provided.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1C each illustrate a dispersion state in a polar solvent;

FIGS. 2A and 2B each illustrate a dispersion state in a polar solvent;

FIGS. 3A to 3C illustrate a positive electrode:

FIG. 4 is a flow chart illustrating a method of forming a positiveelectrode;

FIGS. 5A and 5B illustrate a coin-type secondary battery;

FIGS. 6A and 6B illustrate an electrophoresis method and anelectrochemical reduction method, respectively;

FIG. 7 illustrates a laminated secondary battery;

FIGS. 8A and 8B illustrate a cylindrical secondary battery;

FIG. 9 illustrates electronic devices;

FIGS. 10A to 10C illustrate an electronic device;

FIGS. 11A and 11B illustrate an electronic device;

FIG. 12 shows comparison between charge-discharge characteristics;

FIGS. 13A and 13B show charge-discharge characteristics of a cell A anda cell B;

FIGS. 14A and 14B are SEM images of a positive electrode active materiallayer using a graphene oxide as a raw material of a conductive additive;

FIG. 15 is a SEM image of a positive electrode active material layerusing a graphene oxide as a raw material of a conductive additive;

FIGS. 16A and 16B are SEM images of a positive electrode active materiallayer using a graphene oxide as a raw material of a conductive additive;

FIGS. 17A and 17B are SEM images of a positive electrode active materiallayer using a RGO as a raw material of a conductive additive;

FIGS. 18A and 18B are SEM images of a positive electrode active materiallayer using a graphene as a raw material of a conductive additive;

FIG. 19 illustrates a positive electrode;

FIGS. 20A and 20B are SEM images of a positive electrode active materiallayer using a graphene oxide as a raw material of a conductive additive;

FIG. 21 is a SEM image of a positive electrode active material layerusing a graphene oxide as a raw material of a conductive additive; and

FIG. 22 is a SEM image of a positive electrode active material layerusing a graphene oxide as a raw material of a conductive additive.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments will be described with reference to theaccompanying drawings. However, the embodiments can be implemented inmany different modes, and it will be readily appreciated by thoseskilled in the art that modes and details thereof can be changed invarious ways without departing from the spirit and scope of the presentinvention. Thus, the present invention should not be interpreted asbeing limited to the following description of the embodiments.

Note that in each drawing described in ibis specification, the size, thefilm thickness, or the region of each component is exaggerated forclarity in some cases. Therefore, embodiments of the present inventionare not limited to such scales in the drawings.

Embodiment 1

In this embodiment, a positive electrode for a nonaqueous secondarybattery in accordance with one embodiment of the present invention isdescribed with reference to FIGS. 3A to 3C and FIG. 19. FIG. 3A is aperspective view of the positive electrode, FIG. 3B is a plan view of apositive electrode active material layer, and FIG. 3C and FI. 19 arelongitudinal sectional views of the positive electrode active materiallayer.

FIG. 3A is a perspective view of a positive electrode 200. Although thepositive electrode 200 in the shape of a rectangular sheet isillustrated in FIG. 3A, there is no limitation on the shape of thepositive electrode 200 and any appropriate shape can be selected. Thepositive electrode 200 is formed in such a manner that a positiveelectrode paste is applied on a positive electrode current collector 201and then dried under a reducing atmosphere or reduced pressure to form apositive electrode active material layer 202. The positive electrodeactive material layer 202 is formed over only one surface of thepositive electrode current collector 201 in FIG. 3A but may be formedover both surfaces of the positive electrode current collector 201. Thepositive electrode active material layer 202 is not necessarily formedover the entire surface of the positive electrode current collector 201and a region that is not coated, such as a region for connection to apositive electrode tab, is provided as appropriate.

The positive electrode current collector 201 can be formed using amaterial that has high conductivity and is not alloyed with a carrierion of lithium or the like, such as a metal typified by stainless steel,gold, platinum, zinc, iron, copper, aluminum, or titanium, or an alloythereof. The positive electrode current collector 201 can be formedusing an aluminum alloy to which an element which improves heatresistance, such as silicon, titanium, neodymium, scandium, ormolybdenum, is added. Alternatively, a metal element which formssilicide by reacting with silicon may be used. Examples of the metalelement which forms silicide by reacting with silicon are zirconium,titanium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum,tungsten, cobalt, nickel, and the like. The positive electrode currentcollector 201 can have a foil-like shape, a plate-like shape (sheet-likeshape), a not-like shape, a punching-metal shape, an expanded-metalshape, or the like as appropriate. The positive electrode currentcollector 201 preferably has a thickness greater than or equal to 10 μmand less than or equal to 30 μm.

FIGS. 3B and 3C are schematic views illustrating a top view and alongitudinal section, respectively, of the positive electrode activematerial layer 202. The positive electrode active material layer 202includes positive electrode active material particles 203, graphenes 204as a conductive additive, and a binding agent (also referred to as abinder)(not shown).

The positive electrode active material particle 203 is in the form ofparticles made of secondary particles having average particle diameteror particle diameter distribution, which is obtained in such a way thatmaterial compounds are mixed at a predetermined ratio and baked and theresulting baked product is crushed, granulated, and classified by anappropriate means. Therefore the positive electrode active materialparticles 203 are schematically illustrated as spheres in FIGS. 3B and3C but this shape does not limit the present invention.

As the positive electrode active material particle 203, a materialinto/from which lithium ions can be intercalated/deintercalated can beused; for example, a lithium-containing composite oxide with an olivinecrystal structure, a layered rock-salt crystal structure, or a spinelcrystal structure can be used.

An example of an olivine-type lithium-containing composite oxide isLiMPO₄ (general formula) (M is one or more of Fe(II), Mn(II), Co(II),and Ni(II)). Typical examples of LiMPO₄ (general formula) are LFePO₄,LiNiPO₄, LiCoPO₄, LiMnPO₄, LiFe_(a)Ni_(b)PO₄, LiFe_(a)Co_(b)PO₄,LiFe_(a)Mn_(b)PO₄, LiNi_(a)Co_(b)PO₄, LiNi_(a)Mn_(b)PO₄ (a+b≤1, 0<a<1,and 0<b<1), LiFe_(c)NO_(d)PO₄, LiFe_(c)Ni_(d)Mn_(e)PO₄,LiNi_(c)Co_(d)Mn_(e)PO₄ (c+d+e≤1, 0<c<1, 0<d<1, and 0<e<1),LiFe_(f)Ni_(g)Co_(h)Mn_(i)PO₄ (f+g+h+i≤1, 0<f<1, 0<g<1, 0<h<1, and0<i<1), and the like.

In particular, LiFePO₄ is preferable because it properly satisfiesconditions necessary for the positive electrode active materialparticle, such as safety, stability, high capacity density, highpotential, and the existence of lithium ions which can be extracted ininitial oxidation (charging).

Examples of a lithium-containing composite oxide with a layeredrock-salt crystal structure are lithium cobalt oxide (LiCo₂), LiNiO₂,LiMnO₂, Li₂MnO₃, NiCo-containing composite oxide (general formula:LiNi_(x)Co_(1−x)O₂ (0<x<1)) such as LiNi_(0.8)Co_(0.2)O₂,NiMn-containing composite oxide (general formula: LiNi_(x)Mn_(1−x)O₂(0<x<1)) such as LiNi_(0.5)Mn_(0.5)O₂, NiMnCo-containing composite oxide(also referred to as NMC) (general formula: LiNi_(x)Mn_(y)Co_(1−x−y)O₂(x>0, y>0, x+y<1)) such as LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂,Li(Ni_(0.8)Co_(0.15)Al_(0.05))O₂, Li₂MnO₃—LiMO₂ (M=Co, Ni, or Mn), andthe like.

In particular, LiCoO₂ is preferable because of its advantages such ashigh capacity and stability in the air higher than that of LiNiO₂ andthermal stability higher than that of LiNiO₂.

Examples of a lithium-containing composite oxide with a spinel crystalstructure are LiMn₂O₄, Li_(1+x)Mn_(2−x)O₄, Li(MnAl)₂O₄, andLiMn_(1.5)Ni_(0.5)O₄, and the like.

It is preferable to add a small amount of lithium nickel oxide (LiNiO₂or LiNi_(1−x)MO₂ (M=Co, Al, or the like)) to lithium-containingcomposite oxide with a spinel crystal structure which contains manganesesuch as LiMn₂O₄ because advantages such as minimization of the elutionof manganese and the decomposition of an electrolytic solution can beobtained.

Alternatively, a composite oxide expressed by La_((2−j))MSiO₄ (generalformula) (M is one or more of Fe(II), Mn(II), Co(II), and Ni(II), 0≤j≤2)can be used as the positive electrode active material particle. Typicalexamples of Li_((2−j))MSiO₄ (general formula) are Li_((2−j))FeSiO₄,Li_((2−j))NiSO₄, Li_((2−j))CoSiO₄, Li_((2−j))MnSiO₄,Li_((2−j))Fe_(k)Ni_(l)SiO₄, Li_((2−j))Fe_(k)Co_(l)SiO₄,Li_((2−j))Fe_(k)Mn_(l)SiO₄, Li_((2−j))Ni_(k)Co_(l)SiO₄,Li_((2−j))Ni_(k)Mn_(l)SiO₄ (k+l≤1, 0<k<1, and 0<l<1),L_((2−j))Fe_(m)Ni_(n)Co_(q)SiO₄, L_((2−j))Fe_(m)Ni_(n)Mn_(q)SiO₄,L_((2−j))Ni_(m)Co_(n)Mn_(q)SiO₄ (m+n+q≤1, 0<m<1, 0<n<1, and 0<q<1),Li_((2−j))Fe_(r)Ni_(s)Co_(t)Mn_(u)SiO₄ (r+s+t+u≤1, 0<r<1, 0<s<1, 0<t<1,and 0<u<1), and the like.

Still alternatively, a nasicon compound expressed by A_(x)M₂(XO₄)₃(general formula) (A=Li, Na, or Mg, M=Fe, Mn, Ti, V, Nb, or Al, X=S, P,Mo, W, As, or Si) can be used as the positive electrode active materialparticle. Examples of the nasicon compound are Fe₂(MnO₄)₃, Fe₂(SO₄)₃,Li₃Fe₂(PO₄), and the like. Further alternatively, a compound expressedby Li₂MPO₄F, Li₂MP₂O, or Li₅MO₄ (general formula) (M=Fe or Mn), aperovskite fluoride such as NaFi or FeF₃, a metal chalcogenide (asulfide, a selenide, or a telluride) such as TiS₂ or MoS₂, alithium-containing composite oxide with an inverse spinel crystalstructure such as LiMVO₄, a vanadium oxide (V₂O₅, V₆O₁₃, LiV₃O₈, or thelike), a manganese oxide, an organic sulfur, or the like can be used asthe positive electrode active material particle.

In the case where carrier ions are alkali metal ions other than lithiumions, alkaline-earth metal ions, beryllium ions, or magnesium ions, thepositive electrode active material particle may contain, instead oflithium in the lithium compound and the lithium-containing compositeoxide, an alkali metal (e.g., sodium or potassium), an alkaline-earthmetal (e.g., calcium, strontium, or barium), beryllium, or magnesium.

Further, the graphenes 204 added as a conductive additive to thepositive electrode active material layer 202 are formed by reductiontreatment of a graphene oxide in which the atomic ratio of oxygen tocarbon is greater than or equal to 0.405.

The graphene oxide in which the atomic ratio of oxygen to carbon isgreater than or equal to 0.405 can be formed by an oxidation methodcalled a Hummers method.

The Hummers method is as follows: a sulfuric acid solution of potassiumpermanganate, a hydrogen peroxide solution, or the like is mixed into agraphite powder to cause oxidation reaction; thus, a dispersion liquidincluding a graphite oxide is formed. Through the oxidation of carbon ofgraphite, functional groups such as an epoxy group, a carbonyl group, acarboxyl group, or a hydroxyl group are bonded in the graphite oxide.Accordingly, the interlayer distance between a plurality of graphenes inthe graphite oxide is long as compared to the graphite, so that thegraphite oxide can be easily separated into thin pieces by interlayerseparation. Then, ultrasonic vibration is applied to the dispersionliquid including the graphite oxide, so that the graphite oxide whoseinterlayer distance is long can be cleaved to separate a graphene oxideand to form a dispersion liquid containing a graphene oxide. The solventis removed from the dispersion liquid including the graphene oxide, sothat a powdery graphene oxide can be obtained.

Here, the amount of an oxidizer such as potassium permanganate isadjusted as appropriate so that the graphene oxide in which the atomicratio of oxygen to carbon is greater than or equal to 0.405 can beformed. Specifically, the ratio of the amount of an oxidizer to theamount of a graphite powder is increased, and accordingly the degree ofoxidation of the graphene oxide (the atomic ratio of oxygen to carbon)can be increased. Therefore, in accordance with the amount of thegraphene oxide to be produced, the ratio of the amount of an oxidizer tothe amount of a graphite powder which is a raw material can bedetermined.

For the production of the graphene oxide, the present invention is notlimited to the Hummers method using a sulfuric acid solution ofpotassium permanganate; for example, the Hummers method using nitricacid, potassium chlorate, nitric acid sodium, or the like or a method ofproducing the graphene oxide other than the Hummers method may beemployed as appropriate.

The graphite oxide may be separated into thin pieces by application ofultrasonic vibration, by irradiation with microwaves, radio waves, orthermal plasma, or by application of physical stress.

The formed graphene oxide includes an epoxy group, a carbonyl group, acarboxyl group, a hydroxyl group, or the like. In the graphene oxide,oxygen in a functional group is negatively charged in a polar solventtypified by NMP; therefore, while interacting with NMP, the grapheneoxide repels with other graphene oxides and is hardly aggregated.Accordingly, in a polar solvent, graphene oxides can be easily disperseduniformly.

The length of one side (also referred to as a flake size) of thegraphene oxide is greater than or equal to 50 nm and less than or equalto 100 μm, preferably greater than or equal to 800 nm and less than orequal to 20 μm. Particularly in the case where the flake size is smallerthan the average particle diameter of the positive electrode activematerial particles 203, surface contact with the plurality of positiveelectrode active material particles 203 and connection among graphenesbecome difficult, resulting in difficulty in improving the electronconductivity of the positive electrode active material layer 202.

As in the top view of the positive electrode active material layer 202in FIG. 3B, the plurality of positive electrode active materialparticles 203 is coated with the plurality of graphenes 204. Thesheet-like graphene 204 is connected to the plurality of positiveelectrode active material particles 203. In particular, since thegraphenes 204 are in the form of a sheet, surface contact can be made insuch a way that the graphenes 204 wrap part of surfaces of the positiveelectrode active material particles 203. Unlike a conductive additive inthe form of particles such as acetylene black, which makes point contactwith a positive electrode active material, the graphenes 204 are capableof surface contact with low contact resistance; accordingly, theelectron conductivity of the positive electrode active materialparticles 203 and the graphenes 204 can be improved without an increasein the amount of a conductive additive.

Further, surface contact is made between the plurality of graphenes 204.This is because the graphene oxides with extremely high dispersibilityin a polar solvent are used for the formation of the graphenes 204. Thesolvent is removed by volatilization from a dispersion medium includingthe graphene oxides uniformly dispersed and the graphene oxides arereduced to give the graphenes; hence, the graphenes 204 remaining in thepositive electrode active material layer 202 are partly overlapped witheach other and dispersed such that surface contact is made, therebyforming a path for electron conduction.

In the top view of the positive electrode active material layer 202 inFIG. 3B, the graphenes 204 are not necessarily overlapped with anothergraphene over a surface of the positive electrode active material layer202; the graphenes 204 are formed so as to be three-dimensionallyarranged, for example, so as to enter the inside of the positiveelectrode active material layer 202. Further, the graphenes 204 areextremely thin films (sheets) made of a single layer of carbon moleculesor stacked layers thereof and hence are over and in contact with part ofthe surfaces of the positive electrode active material particles 203 insuch a way as to trace these surfaces. A portion of the graphenes 204which is not in contact with the positive electrode active materialparticles 203 is warped between the plurality of positive electrodeactive material particles 203 and crimped or stretched.

The longitudinal section of the positive electrode active material layer202 shows, as illustrated in FIG. 3C, substantially uniform dispersionof the sheet-like graphenes 204 in the positive electrode activematerial layer 202. The graphenes 204 are schematically shown as heavylines in FIG. 3C but are actually thin films having a thicknesscorresponding to the thickness of a single layer or a multi-layer ofcarbon molecules. As in the top view of the positive electrode activematerial layer 202, the plurality of graphenes 204 are formed in such away as to wrap or coat the plurality of positive electrode activematerial particles 203, so that the graphenes 204 make surface contactwith the positive electrode active material particles 203. Furthermore,the graphenes 204 are also in surface contact with each other;consequently, the plurality of graphenes 204 forms a network forelectron conduction. FIG. 19 is a schematic enlarged view of FIG. 3C.The graphenes 204 coat the surfaces of the plurality of positiveelectrode active material particles 203 in such a way as to cling to thesurfaces and the graphenes are also in contact with each other, and thusthe network is formed.

As illustrated in FIGS. 3B and 3C and FIG. 19, the plurality ofsheet-like graphenes 204 is three-dimensionally dispersed in thepositive electrode active material layer 202 and in surface contact witheach other, which forms the three-dimensional network for electronconduction. Further, each graphene 204 coats and makes surface contactwith the plurality of positive electrode active material particles 203.Thus, bond between the positive electrode active material particles 203is maintained. As described above, the graphenes, whose raw material isthe graphene oxide in which the atomic ratio of oxygen to carbon isgreater than or equal to 0.405 and which are formed by reductionperformed after a paste is formed, are employed as a conductiveadditive, so that the positive electrode active material layer 202 withhigh electron conductivity can be formed.

The proportion of the positive electrode active material particles 203in the positive electrode active material layer 202 can be increasedbecause the added amount of the conductive additive is not necessarilyincreased in order to increase contact points between the positiveelectrode active material particles 203 and the graphenes 204.Accordingly, the discharge capacity of the secondary battery can beincreased.

The average particle diameter of the primary particle of the positiveelectrode active material particles 203 is less than or equal to 500 nm,preferably greater than or equal to 50 nm and less than or equal to 500nm. To make surface contact with the plurality of positive electrodeactive material particles 203, the graphenes 204 have sides the lengthof each of which is greater than or equal to 50 nm and less than orequal to 100 μm, preferably greater than or equal to 800 nm and lessthan or equal to 20 sm.

As the binding agent (binder) included in the positive electrode activematerial layer 202, instead of polyvinylidene fluoride (PVDF) as atypical one, polyimide, polytetrafluoroethylene, polyvinyl chloride,ethylene-propylene-diene polymer, styrene-butadiene rubber,acrylonitrile-butadiene rubber, fluorine rubber, polyvinyl acetate,polymethyl methacrylate, polyethylene, nitrocellulose or the like can beused.

The above-described positive electrode active material layer 202preferably includes the positive electrode active material particles 203at greater than or equal to 90 wt % and less than or equal to 94 wt %,the graphenes 204 as a conductive additive at greater than or equal to 1wt % and less than or equal to 5 wt %, and the binding agent at greaterthan or equal to 1 wt % and less than or equal to 5 wt % with respect tothe total weight of the positive electrode active material layer 202.

As described in this embodiment, the graphenes 204 larger than theaverage particle diameter of the positive electrode active materialparticles 203 are dispersed in the positive electrode active materiallayer 202 such that one of the graphenes 204 makes surface contact withone or more graphenes 204 adjacent to the one of the graphenes 204, andthe graphenes 204 make surface contact in such a way as to wrap thesurfaces of the positive electrode active material particles 203.Consequently, with a small amount of a conductive additive, a positiveelectrode for a nonaqueous secondary battery which is highly filled andincludes a high-density positive electrode active material layer can beprovided.

This embodiment can be implemented combining with another embodiment asappropriate.

Embodiment 2

Next, a method of forming the positive electrode 200 including apositive electrode active material layer 202 is described with referenceto FIG. 4. The method is as follows: a positive electrode paste isformed using the positive electrode active material, the conductiveadditive, the binding agent, and the dispersion medium described above,applied on the positive electrode current collector 201, and then driedunder a reducing atmosphere or reduced pressure.

First, NMP is prepared as the dispersion medium (Step S11), and thegraphene oxide in which the atomic ratio of oxygen to carbon is greaterthan or equal to 0.405 and which is described in Embodiment 1 isdispersed in NMP (Step S12). In the case where the weight of thegraphene oxide is less than 2 wt % with respect to the total weight ofthe positive electrode paste, the conductivity is decreased when thepositive electrode active material layer 202 is formed. In the casewhere the weight of the graphene oxide exceeds 10 wt %, although itdepends on the diameter of the positive electrode active materialparticle, the viscosity of the positive electrode paste is increased. Inaddition, in a drying step after the positive electrode paste is appliedon the positive electrode current collector 201, convection occurs inthe positive electrode paste by heating and the graphene oxide which isthin and lightweight moves and is aggregated, whereby the positiveelectrode active material layer 202 might cause a crack or might beseparated from the positive electrode current collector 201. Thus, theweight of the graphene oxide is preferably set to 2 wt % to 10 wt % withrespect to the weight of the positive electrode paste (the total weightof the positive electrode active material, the conductive additive, andthe binding agent). Note that the graphene oxide is reduced by a laterheat treatment step to give the graphene and the weight is reduced byalmost half, and consequently the weight ratio in the positive electrodeactive material layer 202 becomes 1 wt % to 5 wt %.

Next, lithium iron phosphate is added as the positive electrode activematerial (Step S13). It is preferable to use lithium iron phosphate withan average primary particle diameter greater than or equal to 50 nm andless than or equal to 500 nm. The weight of added lithium iron phosphateis preferably greater than or equal to 85 wt % with respect to the totalweight of the positive electrode paste; for example, the weight isgreater than or equal to 85 wt % and less than or equal to 93 wt %.

Note that when lithium iron phosphate is baked, a carbohydrate such asglucose may be mixed so that a particle of lithium iron phosphate iscoated with carbon. This treatment improves the conductivity.

Next, a mixture of the above is kneaded (mixing is performed in a highlyviscous state), so that the aggregation of the graphene oxide andlithium iron phosphate can be undone. Further, since the graphene oxidehas a functional group, oxygen in the functional group is negativelycharged in a polar solvent, which makes aggregation among differentgraphene oxides difficult. In addition, the graphene oxide stronglyinteracts with lithium iron phosphate. Hence, the graphene oxide can beuniformly dispersed into lithium iron phosphate.

Next, as the binding agent, PVDF is added to the mixture (Step S14). Theweight of PVDF can be determined in accordance with the weight of thegraphene oxide and lithium iron phosphate, and PVDF is preferably addedto the positive electrode paste at greater than or equal to 1 wt % andless than or equal to 5 wt %. The binding agent is added while thegraphene oxide is uniformly dispersed so as to make surface contact withthe plurality of positive electrode active material particles, so thatthe positive electrode active material particles and the graphene oxidecan be bound to each other while the dispersion state is maintained.Although the binding agent is not necessarily added depending on theproportions of lithium iron phosphate and the graphene oxide, adding thebinding agent can enhance the strength of the positive electrode.

Next, NMP is added to this mixture until predetermined viscosity isobtained (Step S15) and mixed. Consequently, the positive electrodepaste can be formed (Step S16). Through the above steps, the positiveelectrode paste in which the graphene oxide, the positive electrodeactive material particles, and the binding agent are uniformly mixed canbe formed.

Next, the positive electrode paste is applied on the positive electrodecurrent collector 201 (Step S7).

Next, the positive electrode paste applied on the positive electrodecurrent collector 201 is dried (Step S18). The drying step is performedby heating at 60° C. to 170° C. for 1 minute to 10 hours to vaporizeNMP. There is no particular limitation on the atmosphere.

Next, the positive electrode paste is dried under a reducing atmosphereor reduced pressure (Step S19). By heating at a temperature of 130° C.to 200° C. for 10 hours to 30 hours under a reducing atmosphere orreduced pressure, NMP and water which are left in the positive electrodepaste are vaporized and oxygen contained in the graphene oxide isdesorbed. Thus, the graphene oxide can be formed into graphene. Notethat oxygen in the graphene oxide may partly remain in the graphenewithout being entirely released.

Through the above steps, the positive electrode 200 including thepositive electrode active material layer 202 where the graphenes 204 areuniformly dispersed in the positive electrode active material particles203 can be formed. Note that a step of applying pressure to the positiveelectrode 200 may be performed after the drying step.

As described in this embodiment, the graphene oxide can be uniformlydispersed in positive electrode active material particles by adding thepositive electrode active material particles to a dispersion medium inwhich the graphene oxide with an atomic ratio of oxygen to carbongreater than or equal to 0.405 is dispersed and mixed. By being added ina state where the graphene oxide is dispersed so as to be in contactwith the plurality of the positive electrode active material particles,the binding agent can be uniformly dispersed without hindering thecontact between the graphene oxide and the plurality of positiveelectrode active material particles. With use of the positive electrodepaste formed in such a manner, a positive electrode which is highlyfilled with the positive electrode active material and includes ahigh-density positive electrode active material layer can bemanufactured. Further, when a battery is formed using the positiveelectrode, a nonaqueous secondary battery with high capacity can beformed. Since a state where the sheet-like graphenes are in contact withthe plurality of positive electrode active material particles can bemaintained by the binding agent, separation between the positiveelectrode active material and the graphene can be suppressed; thus, anonaqueous secondary battery having good cycle characteristics can bemanufactured.

This embodiment can be implemented combining with another embodiment asappropriate.

Embodiment 3

In this embodiment, a structure of a nonaqueous secondary battery and amanufacturing method thereof will be described with reference to FIGS.5A and 5B and FIGS. 6A and 6B.

FIG. 5A is an external view of a coin-type (single-layer flat type)nonaqueous secondary battery, and FIG. 5B is a cross-sectional viewthereof.

In a coin-type secondary battery 300, a positive electrode can 301serving also as a positive electrode terminal and a negative electrodecan 302 serving also as a negative electrode terminal are insulated andsealed with a gasket 303 formed of polypropylene or the like. A positiveelectrode 304 is formed of a positive electrode current collector 305and a positive electrode active material layer 306 which is provided tobe in contact with the positive electrode current collector 305. On theother hand, a negative electrode 307 is formed of a negative electrodecurrent collector 308 and a negative electrode active material layer 309which is provided to be in contact with the negative electrode currentcollector 308. A separator 310 and an electrolyte (not illustrated) areincluded between the positive electrode active material layer 306 andthe negative electrode active material layer 309.

As the positive electrode 304, the positive electrode 200 described inEmbodiment 1 and Embodiment 2 can be used.

The negative electrode 307 can be formed in such a manner that thenegative electrode active material layer 309 is formed over the negativeelectrode current collector 308 by a CVD method, a sputtering method, ora coating method.

For the negative electrode current collector 308, it is possible to usea highly conductive material, for example, a metal such as aluminum,copper, nickel, or titanium, an aluminum-nickel alloy, or analuminum-copper alloy. The negative electrode current collector 308 canhave a foil-like shape, a plate-like shape (a sheet-like shape), anet-like shape, a punching-metal shape, an expanded-metal shape, or thelike as appropriate. The negative electrode current collector 308preferably has a thickness of greater than or equal to 10 μm and lessthan or equal to 30 μm.

As the negative electrode active material, a material with which lithiumcan be dissolved/precipitated or a material into/from which lithium ionscan be intercalated/deintercalated can be used; for example, a lithiummetal, a carbon-based material, an alloy-based material, or the like canbe used.

The lithium metal is preferable because of its low redox potential(lower than that of the standard hydrogen electrode by 3.045 V) and highspecific capacity per weight and volume (which are 3860 mAh/g and 2062mAh/cm³).

Examples of the carbon-based material include graphite, graphitizingcarbon (soft carbon), non-graphitizing carbon (hard carbon), a carbonnanotube, graphene, carbon black, and the like.

Examples of the graphite include artificial graphite such as meso-carbonmicrobeads (MCMB), coke-based artificial graphite, and pitch-basedartificial graphite and natural graphite such as spherical naturalgraphite.

Graphite has a low potential substantially equal to that of a lithiummetal (0.1 V to 0.3 V vs. Li/Li⁺) when lithium ions are intercalatedinto the graphite (when a lithium-graphite intercalation compound isgenerated). For this reason, a lithium ion battery can have a highoperating voltage. In addition, graphite is preferable because of itsadvantages such as relatively high capacity per volume, small volumeexpansion, low cost, and greater safety than that of a lithium metal.

As the negative electrode active material, an alloy-based material whichenables charge-discharge reaction by alloying and dealloying reactionwith a lithium metal can be used. For example, a material including atleast one of Al, Si, Ge, Sn, Pb, Sb, Bi, Ag, Zn, Cd, In, Ga, and thelike can be given. Such elements have higher capacity than carbon. Inparticular, silicon has a theoretical capacity of 4200 mAh/g, which issignificantly high. For this reason, silicon is preferably used as thenegative electrode active material. Examples of the alloy-based materialusing such elements include SiO, MgSi, Mg₂Ge, SnO, SnOz, Mg₂Sn, SnS₂,VSn₃, FeSn₂, CoSm, Ni₃Sn₂, CuSns, AgSn, Ag₃Sb, Ni₂MnSb, CeSbs, LaSn₃,LaCouSn₂, CoSb₃, InSb, SbSn, and the like.

Alternatively, as the negative electrode active material, an oxide suchas titanium dioxide (TiO₂), lithium titanium oxide (Li₄Ti₅O₁₂), alithium-graphite intercalation compound (LiC₆), niobium pentoxide(Nb₂O₅), tungsten oxide (WO₂), molybdenum oxide (MoO₂), or the like canbe used.

Further alternatively, as the negative electrode active material,L_(3−x)M_(x)N (M=Co, Ni, or Cu) with a Li₃N structure, which is anitride containing lithium and a transition metal, can be used. Forexample, Li_(2.6)Co_(0.4)N₃ is preferable because of high charge anddischarge capacity (900 mAh/g).

A nitride containing lithium and a transition metal is preferably used,in which case lithium ions are included in the negative electrode activematerial, and thus the negative electrode active material can be used incombination with a material for a positive electrode active materialwhich does not include lithium ions, such as V₂O₅ or Cr₃O₈. Note that inthe case of using a material including lithium ions as the positiveelectrode active material, the nitride containing lithium and atransition metal can be used for the negative electrode active materialby extracting lithium ions in advance.

Still further alternatively, as the negative electrode active material,a material which causes conversion reaction can be used. For example, atransition metal oxide which does not cause alloying reaction withlithium, such as cobalt oxide (COO), nickel oxide (NiO), or iron oxide(FoO), may be used. Other examples of the material which causesconversion reaction include oxides such as FezO₃, CuO, Cu₂O, RuO₂, andCr₂O₃, sulfides such as CoSo_(0.89), NiS, and CuS, nitrides such asZn₃N₂, Cu₃N, and Ge₃N₄, phosphides such as NiP₂, FeP₂, and CoP₃, andfluorides such as FeF₃ and BiF₃. Note that any of the fluorides can beused as a positive electrode active material because of its highpotential.

The negative electrode active material layer 309 may be formed by acoating method in the following manner: a conductive additive or abinding agent is added to the negative electrode active material to forma negative electrode paste; and the negative electrode paste is appliedon the negative electrode current collector 308 and dried.

In the case where the negative electrode active material layer 309 isformed using silicon as the negative electrode active material, grapheneis preferably formed on a surface of the negative electrode activematerial layer 309. The volume of silicon is greatly changed due toocclusion/release of carrier ions in charge-discharge cycles, adhesionbetween the negative electrode current collector 308 and the negativeelectrode active material layer 309 is decreased, resulting indegradation of battery characteristics caused by charge and discharge.In view of this, graphene is preferably formed on a surface of thenegative electrode active material layer 309 containing silicon becauseeven when the volume of silicon is changed in charge-discharge cycles,decrease in adhesion between the negative electrode current collector308 and the negative electrode active material layer 309 can besuppressed and degradation of battery characteristics is reduced.

Graphene formed on the surface of the negative electrode active materiallayer 309 can be formed by reducing graphene oxide in a similar mannerto that of the method of forming the positive electrode. As the grapheneoxide, the graphene oxide described in Embodiment 1 can be used.

A method of forming graphene oxide on the negative electrode activematerial layer 309 by an electrophoresis method will be described withreference to FIG. 6A.

FIG. 6A is a cross-sectional view illustrating an electrophoresismethod. In a container 401, the dispersion liquid in which grapheneoxide is dispersed and which is described in Embodiment 1 (hereinafterreferred to as a graphene oxide dispersion liquid 402) is contained.Further, a formation subject 403 is put in the graphene oxide dispersionliquid 402 and is used as an anode. In addition, a conductor 404 servingas a cathode is put in the graphene oxide dispersion liquid 402. Notethat the formation subject 403 is the negative electrode currentcollector 308 and the negative electrode active material layer 309 whichis formed thereon. Further, the conductor 404 may be formed using aconductive material, for example, a metal material or an alloy material.

By applying appropriate voltage between the anode and the cathode, agraphene oxide layer is formed on a surface of the formation subject403, that is, the surface of the negative electrode active materiallayer 309. This is because the graphene oxide is negatively charged inthe polar solvent as described above, so that by applying voltage, thegraphene oxide which is negatively charged is drawn to the anode anddeposited on the formation subject 403. Negative charge of the grapheneoxide is derived from release of hydrogen ions from a substituent suchas a hydroxyl group or a carboxyl group included in the graphene oxide,and the substituent is bonded to an object to result in neutralization.Note that the voltage which is applied is not necessarily constant.Further, by measuring the amount of charge flowing between the anode andthe cathode, the thickness of the graphene oxide layer deposited on theobject can be estimated.

The voltage is applied between the cathode and the anode in the range of0.5 V to 2.0 V, preferably 0.8 V to 1.5 V. For example, when the voltageapplied between the cathode and the anode is set to 1 V, an oxide filmwhich might be generated based on the principle of anodic oxidation isnot easily formed between the formation subject and the graphene oxidelayer.

When the graphene oxide with a required thickness is obtained, theformation subject 403 is taken out of the graphene oxide dispersionliquid 402 and dried.

In electrodeposition of the graphene oxide by an electrophoresis method,a portion which is already coated with the graphene oxide is scarcelystacked with an additional graphene oxide. This is because theconductivity of the graphene oxide is sufficiently low. On the otherhand, a portion which is not coated yet with the graphene oxide ispreferentially stacked with graphene oxide. Therefore, the grapheneoxide formed on the surface of the formation subject 403 has a uniformthickness sufficient for practical use.

Time for performing electrophoresis (time for applying voltage) ispreferably longer than time for coating the surface of the formationsubject 403 with the graphene oxide, for example, longer than or equalto 0.5 minutes and shorter than or equal to 30 minutes, more preferablylonger than or equal to 5 minutes and shorter than or equal to 20minutes.

With the use of an electrophoresis method, an ionized graphene oxide canbe electrically transferred to the active material, whereby the grapheneoxide can be provided uniformly even when the surface of the negativeelectrode active material layer 309 is uneven.

Next, part of oxygen is released from the formed graphene oxide byreduction treatment. Although, as the reduction treatment, reductiontreatment by heating or the like, which is described in Embodiment 1using a graphene, may be performed, electrochemical reduction treatment(hereinafter, referred to as electrochemical reduction) will bedescribed below.

The electrochemical reduction of the graphene oxide is reductionutilizing electric energy, which is different from reduction by heattreatment. As illustrated in FIG. 6B, a closed circuit is configuredusing, as a conductor 407, the negative electrode including the grapheneoxide provided over the negative electrode active material layer 309,and a potential at which the reduction reaction of the graphene oxideoccurs or a potential at which the graphene oxide is reduced is suppliedto the conductor 407, so that the graphene oxide is reduced to formgraphene. Note that in this specification, a potential at which thereduction reaction of the graphene oxide occurs or a potential at whichthe graphene oxide is reduced is referred to as the reduction potential.

A method for reducing the graphene oxide will be specifically describedwith reference to FIG. 6B. A container 405 is filled with an electrolytesolution 406, and the conductor 407 provided with the graphene oxide anda counter electrode 408 are put in the container 405 so as to beimmersed in the electrolyte solution 406. Next, an electrochemical cell(open circuit) is configured using at least the counter electrode 408and the electrolyte solution 406 besides the conductor 407 provided withthe graphene oxide, which serves as a working electrode, and thereduction potential of the graphene oxide is supplied to the conductor407 (working electrode), so that the graphene oxide is reduced to formgraphene. Note that the reduction potential to be supplied is areduction potential in the case where the potential of the counterelectrode 408 is used as a reference potential or a reduction potentialin the case where a reference electrode is provided in theelectrochemical cell and the potential of the reference electrode isused as a reference potential. For example, when the counter electrode408 and the reference electrode are each made of lithium metal, thereduction potential to be supplied is a reduction potential determinedrelative to the redox potential of the lithium metal (vs. Li/Li⁺).Through this step, reduction current flows through the electrochemicalcell (closed circuit) when the graphene oxide is reduced. Thus, toexamine whether the graphene oxide is reduced, the reduction currentneeds to be checked continuously; the state where the reduction currentis below a certain value (where there is no peak corresponding to thereduction current) is regarded as the state where the graphene oxide isreduced (where the reduction reaction is completed).

In controlling the potential of the conductor 407, the potential of theconductor 407 may be fixed to less than or equal to the reductionpotential of the graphene oxide or may be swept so as to include thereduction potential of the graphene oxide. Further, the sweeping may beperiodically repeated like in cyclic voltammetry. There is no limitationon the sweep rate of the potential of the conductor 407. Note that thepotential of the conductor 407 may be swept either from a higherpotential to a lower potential or from a lower potential to a higherpotential.

Although the reduction potential of the graphene oxide slightly variesdepending on the structure of the graphene oxide (e.g., the presence orabsence of a functional group) and the way to control the potential(e.g., the sweep rate), it is approximately 2.0 V (vs. Li/Li⁺),Specifically, the potential of the conductor 407 may be controlled so asto fall within the range of 1.6 V to 2.4 V (vs. Li/Li⁺).

Through the above steps, the graphene can be formed over the conductor407. In the case where electrochemical reduction treatment is performed,a proportion of C(sp²)-C(sp²) double bonds is higher than that of thegraphene formed by heat treatment; therefore, the graphene having highconductivity can be formed over the negative electrode active materiallayer 309.

The negative electrode active material layer 309 may be predoped withlithium through the graphene after the graphene is formed over theconductor 407. As a predoping method of lithium, a lithium layer may beformed on a surface of the negative electrode active material layer 309by a sputtering method. Alternatively, a lithium foil is provided on thesurface of the negative electrode active material layer 309, whereby thenegative electrode active material layer 309 can be predoped withlithium.

The separator 310 can be formed using an insulator such as cellulose(paper), polyethylene with pores, or polypropylene with pores.

As an electrolyte of the electrolyte solution, a material which containscarrier ions is used. Typical examples of the electrolyte includelithium salts such as LiClO₄, LiAsF₆, LiBF₄, LiPF₆, and Li(C₂F₅SO₂)N.

In the case where carrier ions are alkali metal ions other than lithiumions, alkaline-earth metal ions, beryllium ions, or magnesium ions, theelectrolyte may contain, instead of lithium in the lithium salts, analkali metal (e.g., sodium or potassium), an alkaline-earth metal (e.g.,calcium, strontium, or barium) beryllium, or magnesium.

As a solvent of the electrolyte solution, a material in which carrierions can transfer is used. As the solvent of the electrolyte solution,an aprotic organic solvent is preferably used, Typical examples ofaprotic organic solvents include ethylene carbonate (EC), propylenecarbonate, dimethyl carbonate, diethyl carbonate (DEC), γ-butyrolactone,acetonitrile, dimethoxyethane, and tetrahydrofuran, and one or more ofthese materials can be used. When a gelled polymer material is used asthe solvent of the electrolyte solution, safety against liquid leakageand the like is improved. Further, the nonaqueous secondary battery canbe thinner and more lightweight. Typical examples of a gelled polymermaterial include a silicone gel, an acrylic gel, an acrylonitrile gel,polyethylene oxide, polypropylene oxide, and a fluorine-based polymer.Alternatively, the use of one or more of ionic liquids (room temperaturemolten salts) which are less likely to burn and volatilize as thesolvent of the electrolyte solution can prevent the secondary batteryfrom exploding or catching fire even when the secondary batteryinternally shorts out or the internal temperature increases due toovercharging or the like.

Instead of the electrolyte solution, a solid electrolyte including asulfide-based inorganic material, an oxide-based inorganic material, orthe like, or a solid electrolyte including a polyethylene oxide(PEO)-based polymer material or the like can be used. When the solidelectrolyte is used, a separator or a spacer is not necessary. Further,the battery can be entirely solidified; therefore, them is nopossibility of liquid leakage and thus the safety of the battery isdramatically increased.

For the positive electrode can 301 and the negative electrode can 302, ametal having a corrosion-resistance property to a liquid (e.g., anelectrolyte solution) in charging and discharging the secondary battery,such as nickel, aluminum, or titanium; an alloy of any of the metals; analloy containing any of the metals and another metal (e.g., stainlesssteel); a stack of any of the metals a stack including any of the metalsand any of the alloys (e.g., a stack of stainless steel and aluminum);or a stack including any of the metals and another metal (e.g., a stackof nickel, iron, and nickel) can be used. The positive electrode can 301and the positive electrode 304 are electrically connected to each other,and the negative electrode can 302 and the negative electrode 307 areelectrically connected to each other.

The negative electrode 307, the positive electrode 304, and theseparator 310 are immersed in the electrolyte. Then, as illustrated inFIG. 5B, the positive electrode 304, the separator 310, the negativeelectrode 307, and the negative electrode can 302 are stacked in thisorder with the positive electrode can 301 positioned at the bottom, andthe positive electrode can 301 and the negative electrode can 302 aresubjected to pressure bonding with the gasket 303 interposedtherebetween. In such a manner, the coin-type secondary battery 300 ismanufactured.

Next, an example of a laminated secondary battery will be described withreference to FIG. 7.

A laminated secondary battery 500 illustrated in FIG. 7 includes apositive electrode 503 including a positive electrode current collector501 and, a positive electrode active material layer 502, a negativeelectrode 506 including a negative electrode current collector 504 and anegative electrode active material layer 505, a separator 507, anelectrolyte solution 508, and an exterior body 509. The separator 507 isplaced between the positive electrode 503 and the negative electrode 506provided in the exterior body 509. The exterior body 509 is filled withthe electrolyte solution 508.

In the laminated secondary battery 500 illustrated in FIG. 7, thepositive electrode current collector 501 and the negative electrodecurrent collector 504 also function as terminals for electrical contactwith the outside. For this reason, each of the positive electrodecurrent collector 501 and the negative electrode current collector 504is arranged outside the exterior body 509 so as to be partly exposed.

In the laminated secondary battery 500, as the exterior body 509, forexample, a laminate film having a three-layer structure where a highlyflexible metal thin film of aluminum, stainless steel, copper, nickel,or the like is provided over a film formed of a material such aspolyethylene, polypropylene, polycarbonate, ionomer, or polyamide, andan insulating synthetic resin film of a polyamide resin, a polyesterresin, or the like is provided as the outer surface of the exterior bodyover the metal thin film can be used. With such a three-layer structure,permeation of an electrolytic solution and a gas can be blocked and aninsulating property and resistance to the electrolytic solution can beobtained.

Next, examples of a cylindrical secondary battery are described withreference to FIGS. 8A and 8B. As illustrated in FIG. 8A, a cylindricallithium secondary battery 600 includes a positive electrode cap (batterylid) 601 on its top surface and a battery can (exterior can) 602 on itsside surface and bottom surface. The positive electrode cap and thebattery can (exterior can) 602 are insulated from each other by a gasket(insulating gasket) 610.

FIG. 8B is a diagram schematically illustrating a cross section of thecylindrical nonaqueous secondary battery. In the battery can 602 with ahollow cylindrical shape, a battery element is provided in which astrip-like positive electrode 604 and a strip-like negative electrode606 are wound with a separator 605 provided therebetween. Although notillustrated, the battery element is wound around a center pin as acenter. One end of the battery can 602 is close and the other endthereof is open. For the battery can 602, a metal having acorrosion-resistance property to a liquid (e.g., an electrolytesolution) in charging and discharging the secondary battery, such asnickel, aluminum, or titanium; an alloy of any of the metals; an alloycontaining any of the metals and another metal (e.g., stainless steel);a stack of any of the metals; a stack including any of the metals andany of the alloys (e.g., a stack of stainless steel and aluminum); or astack including any of the metals and another metal (e.g., a stack ofnickel, iron, and nickel) can be used. Inside the battery can 602, thebattery element in which the positive electrode, the negative electrode,and the separator are wound is interposed between a pair of insulatingplates 608 and 609 which face each other. Further, a non-aqueouselectrolyte solution (not illustrated) is injected inside the batterycan 602 in which the battery element is provided. A non-aqueouselectrolyte solution which is similar to that of the coin-typenonaqueous secondary or the laminated nonaqueous secondary battery canbe used.

Although the positive electrode 604 and the negative electrode 606 canbe formed in a manner similar to that of the positive electrode and thenegative electrode of the coin-type nonaqueous secondary battery, thedifference lies in that, since the positive electrode and the negativeelectrode of the cylindrical nonaqueous secondary battery are wound,active materials are formed on both sides of the current collectors. Apositive electrode terminal (positive electrode current collecting lead)603 is connected to the positive electrode 604, and a negative electrodeterminal (negative electrode current collecting lead) 607 is connectedto the negative electrode 606. A metal material such as aluminum can beused for both the positive electrode terminal 603 and the negativeelectrode terminal 607. The positive electrode terminal 603 isresistance-welded to a safety valve mechanism 612, and the negativeelectrode terminal 607 is resistance-welded to the bottom of the batterycan 602. The safety valve mechanism 612 is electrically connected to thepositive electrode cap 601 through a positive temperature coefficient(PTC) element 611. The safety valve mechanism 612 cuts off electricalconnection between the positive electrode cap 601 and the positiveelectrode 604 when the internal pressure of the battery increases andexceeds a predetermined threshold value. The PTC element 611 is a heatsensitive resistor whose resistance increases as temperature rises, andcontrols the amount of current by increase in resistance to preventunusual beat generation. Barium titanate (BaTiO₃)-based semiconductorceramic or the like can be used for the PTC element.

Note that in this embodiment, the coin-type non-aqueous secondarybattery, the laminated nonaqueous secondary battery, and the cylindricalnon-aqueous secondary battery are given as examples of the lithiumsecondary battery; however, any of non-aqueous secondary batteries withthe other various shapes, such as a sealing-type non-aqueous secondarybattery and a square-type non-aqueous secondary battery, can be used.Further, a structure in which a plurality of positive electrodes, aplurality of negative electrodes, and a plurality of separators arestacked or wound may be employed.

A positive electrode according to one embodiment of the presentinvention is used as each of the positive electrodes of the secondarybatteries 300, 500, and 600 described in this embodiment. Thus, thedischarge capacity of each of the secondary batteries 300, 500, and 600can be increased.

This embodiment can be implemented combining with another embodiment asappropriate.

Embodiment 4

A nonaqueous secondary battery of one embodiment of the presentinvention can be used for power supplies of a variety of electricalappliances.

Specific examples of electrical appliances each utilizing the nonaqueoussecondary battery of one embodiment of the present invention are asfollows: display devices of televisions, monitors, and the like,lighting devices, desktop personal computers and laptop personalcomputers, word processors, image reproduction devices which reproducestill images or moving images stored in recording media such as digitalversatile discs (DVDs), portable compact disc (CD) players, radioreceivers, tape recorders, headphone stereos, stereos, clocks such astable clocks and wall clocks, cordless phone handsets, transceivers,portable wireless devices, cellular phones, car phones, portable gamemachines, calculators, portable information terminals, electronicnotebooks, e-book readers, electronic translators, audio input devices,cameras such as still cameras and video cameras, toy, electric shavers,high-frequency heating appliances such as microwave ovens, electric ricecookers, electric washing machines, electric vacuum cleaners, waterheaters, electric fans, hair dryers, air-conditioning systems such asair conditioners, humidifiers, and dehumidifiers, dishwashers, dishdryers, clothes dryers, futon dryers, electric refrigerators, electricfreezers, electric refrigerator-freezers, freezers for preserving DNA,flashlights, electric power tools such as chain saws, smoke detectors,and medical equipment such as dialyzers. Further, industrial equipmentsuch as guide lights, traffic lights, belt conveyors, elevators,escalators, industrial robots, power storage systems, and power storagedevices for leveling the amount of power supply and smart grid can begiven. In addition, moving objects driven by electric motors using powerfrom the nonaqueous secondary batteries are also included in thecategory of electrical appliances. Examples of the moving objects areelectric vehicles (EV), hybrid electric vehicles (HEV) which includeboth an internal-combustion engine and a motor, plug-in hybrid electricvehicles (PHEV), tracked vehicles in which caterpillar tracks aresubstituted for wheels of these vehicles, motorized bicycles includingmotor-assisted bicycles, motorcycles, electric wheelchairs, golf carts,boats or ships, submarines, helicopters, aircrafts, rockets, artificialsatellites, space probes, planetary probes, and spacecrafts.

In the above electrical appliances, the nonaqueous secondary battery ofone embodiment of the present invention can be used as a main powersupply for supplying enough power for almost the whole powerconsumption. Alternatively, in the above electrical appliances, thenonaqueous secondary battery of one embodiment of the present inventioncan be used as an uninterruptible power supply which can supply power tothe electrical appliances when the supply of power from the main powersupply or a commercial power supply is stopped. Still alternatively, inthe above electrical appliances, the nonaqueous secondary battery of oneembodiment of the present invention can be used as an auxiliary powersupply for supplying power to the electrical appliances at the same timeas the power supply from the main power supply or a commercial powersupply.

FIG. 9 illustrates specific structures of the above electricalappliances. In FIG. 9, a display device is an example of an electricalappliance including a nonaqueous secondary battery 704 of one embodimentof the present invention. Specifically, the display device 700corresponds to a display device for TV broadcast reception and includesa housing 701, a display portion 702, speaker portions 703, thenonaqueous secondary battery 704, and the like. The nonaqueous secondarybattery 704 of one embodiment of the present invention is provided inthe housing 701. The display device 700 can receive power from acommercial power supply. Alternatively, the display device 700 can usepower stored in the nonaqueous secondary battery 704. Thus, the displaydevice 700 can be operated with the use of the nonaqueous secondarybattery 704 of one embodiment of the present invention as anuninterruptible power supply even when power cannot be supplied from acommercial power supply due to power failure or the like.

For the display portion 702, a semiconductor display device such as aliquid crystal display device, a light-emitting device in which alight-emitting element such as an organic EL element is provided in eachpixel, an electrophoresis display device, a digital micromirror device(DMD), a plasma display panel (PDP), or a field emission display (FED)can be used.

Note that the display device includes, in its category, all ofinformation display devices for personal computers, advertisementdisplays, and the like besides TV broadcast reception.

In FIG. 9, an installation lighting device 710 is an example of anelectrical appliance including a nonaqueous secondary battery 713 of oneembodiment of the present invention. Specifically, the lighting device710 includes a housing 711, a light source 712, the nonaqueous secondarybattery 713, and the like. Although FIG. 9 illustrates the case wherethe nonaqueous secondary battery 713 is provided in a ceiling 714 onwhich the housing 711 and the light source 712 are installed, thenonaqueous secondary battery 713 may be provided in the housing 711. Thelighting device 710 can receive power from a commercial power supply.Alternatively, the lighting device 710 can use power stored in thenonaqueous secondary battery 713. Thus, the lighting device 710 can beoperated with the use of the nonaqueous secondary battery 713 of oneembodiment of the present invention as an uninterruptible power supplyeven when power cannot be supplied from a commercial power supply due topower failure or the like.

Note that although the installation lighting device 710 provided in theceiling 714 is illustrated in FIG. 9 as an example, the nonaqueoussecondary battery of one embodiment of the present invention can be usedas an installation lighting device provided in, for example, a wall 715,a floor 716, a window 717, or the like other than the ceiling 714.Alternatively, the nonaqueous secondary battery can be used in atabletop lighting device or the like.

As the light source 712, an artificial light source which emits lightartificially by using power can be used. Specifically, an incandescentlamp, a discharge lamp such as a fluorescent lamp, and light-emittingelements such as an LED or an organic EL element are given as examplesof the artificial light source.

In FIG. 9, an air conditioner including an indoor unit 720 and anoutdoor unit 724 is an example of an electrical appliance including anonaqueous secondary battery 723 of one embodiment of the presentinvention. Specifically, the indoor unit 720 includes a housing 721, anair outlet 722, the nonaqueous secondary battery 723, and the like.Although FIG. 9 illustrates the case where the nonaqueous secondarybattery 723 is provided in the indoor unit 720, the nonaqueous secondarybattery 723 may be provided in the outdoor unit 724. Alternatively, thenonaqueous secondary batteries 723 may be provided in both the indoorunit 720 and the outdoor unit 724. The air conditioner can receive powerfrom a commercial power supply. Alternatively, the air conditioner canuse power stored in the nonaqueous secondary battery 723. Particularlyin the case where the nonaqueous secondary batteries 723 are provided inboth the indoor unit 720 and the outdoor unit 724, the air conditionercan be operated with the use of the nonaqueous secondary battery 723 ofone embodiment of the present invention as an uninterruptible powersupply even when power cannot be supplied from a commercial power supplydue to power failure or the like.

Note that although the split-type air conditioner including the indoorunit and the outdoor unit is illustrated in FIG. 9 as an example, thenonaqueous secondary battery of one embodiment of the present inventioncan be used in an air conditioner in which the functions of an indoorunit and an outdoor unit are integrated in one housing.

In FIG. 9, an electric refrigerator-freezer 730 is an example of anelectrical appliance including a nonaqueous secondary battery 734 of oneembodiment of the present invention. Specifically, the electricrefrigerator-freezer 730 includes a housing 731, a door 732 for therefrigerator, a door 733 for the freezer, the nonaqueous secondarybattery 734, and the like. The nonaqueous secondary battery 734 isprovided in the housing 731 in FIG. 9. The electric refrigerator-freezer730 can receive power from a commercial power supply. Alternatively, theelectric refrigerator-freezer 730 can use power stored in the nonaqueoussecondary battery 734. Thus, the electric refrigerator-freezer 730 canbe operated with the use of the nonaqueous secondary battery 734 of oneembodiment of the present invention as an uninterruptible power supplyeven when power cannot be supplied from a commercial power supply due topower failure or the like.

Note that among the electrical appliances described above, ahigh-frequency heating apparatus such as a microwave oven and anelectrical appliance such as an electric rice cooker require high powerin a short time. The tripping of a breaker of a commercial power supplyin use of an electrical appliance can be prevented by using thenonaqueous secondary battery of one embodiment of the present inventionas an auxiliary power supply for supplying power which cannot besupplied enough by the commercial power supply.

In addition, in a time period when electrical appliances are not used,particularly when the proportion of the amount of power which isactually used to the total weight of power which can be supplied from acommercial power supply source (such a proportion referred to as a usagerate of power) is low, power can be stored in the nonaqueous secondarybattery, whereby the usage rate of power can be reduced in a time periodwhen the electrical appliances are used. For example, in the case of theelectric refrigerator-freezer 730, power can be stored in the nonaqueoussecondary battery 734 in night time when the temperature is low and thedoor 732 for the refrigerator and the door 733 for the freezer are notoften opened and closed. On the other hand, in daytime when thetemperature is high and the door 732 for the refrigerator and the door733 for the freezer am frequently opened and closed, the nonaqueoussecondary battery 734 is used as an auxiliary power supply, thus, theusage rate of power in daytime can be reduced.

This embodiment can be implemented combining with another embodiment asappropriate.

Embodiment 5

Next, a portable information terminal which is an example of theelectrical appliance will be described with reference to FIGS. 10A to10C.

FIGS. 10A and 10B illustrate a tablet terminal 800 that can be folded.In FIG. 10A, the tablet terminal 800 is opened, and includes a housing801, a display portion 802 a, a display portion 802 b, a switch 803 forswitching display modes, a power switch 804, a switch 805 for switchingto power-saving mode, and an operation switch 807.

Part of the display portion 802 a can be a touch panel region 808 a anddata can be input when a displayed operation key 809 is touched.Although a structure in which a half region in the display portion 802 ahas only a display function and the other half region has a touch panelfunction is shown as an example, the display portion 802 a is notlimited to the structure. The whole region in the display portion 802 amay have a touch panel function. For example, keyboard buttons can bedisplayed on the entire display portion 802 a to be used as a touchpanel, and the display portion 802 b can be used as a display screen.

As in the display portion 802 a, part of the display portion 802 b canbe a touch panel region 808 b. A switching button 810 for showing/hidinga keyboard of the touch panel is touched with a finger, a stylus, or thelike, so that keyboard buttons can be displayed on the display portion802 b.

Touch input can be performed in the touch panel region 808 a and thetouch panel region 808 b at the same time.

The switch 803 for switching display modes can switch the displaybetween portrait mode, landscape mode, and the like, and betweenmonochrome display and color display, for example. The switch 805 forswitching to power-saving mode can control display luminance to beoptimal in accordance with the amount of external light in use of thetablet terminal which is detected by an optical sensor incorporated inthe tablet terminal. Another detection device including a sensor or thelike for detecting inclination, such as a gyroscope or an accelerationsensor, may be incorporated in the tablet terminal, in addition to theoptical sensor.

Note that FIG. 10A illustrates an example in which the display portion802 a and the display portion 802 b have the same display area: howeverwithout limitation thereon, one of the display portions may be differentfrom the other display portion in size and display quality. For example,one display panel may be capable of higher-definition display than theother display panel.

The tablet terminal 800 is closed in FIG. 10B. The tablet terminalincludes the housing 801, a solar cell 811, a charge-discharge controlcircuit 850, a battery 851, and a DC-DC converter 852. In FIG. 10B, astructure including the battery 851 and the DC-DC converter 852 isillustrated as an example of the charge-discharge control circuit 850.The nonaqueous secondary battery described in any of the aboveembodiments is used as the battery 851.

Since the tablet terminal 800 can be folded, the housing 801 can beclosed when the tablet terminal is not used. As a result, the displayportion 802 a and the display portion 802 b can be protected; thus, thetablet terminal 800 which has excellent durability and excellentreliability also in terms of long-term use can be provided.

In addition, the tablet terminal illustrated in FIGS. 10A and 10B canhave a function of displaying a variety of kinds of data (e.g., a stillimage, a moving image, and a text image), a function of displaying acalendar, a date, the time, or the like on the display portion, atouch-input function of operating or editing the data displayed on thedisplay portion by touch input, a function of controlling processing bya variety of kinds of software (programs), and the like.

The solar cell 811 provided on a surface of the tablet terminal cansupply power to the touch panel, the display portion, a video signalprocessing portion, or the like. Note that the solar cell 811 can bepreferably provided on one or both surfaces of the housing 801, in whichcase the battery 851 can be charged efficiently. When the nonaqueoussecondary battery described in any of the above embodiments is used asthe battery 851, there is an advantage such as a reduction in size.

The structure and the operation of the charge-discharge control circuit850 illustrated in FIG. 10B will be described with reference to a blockdiagram in FIG. 10C. The solar cell 811, the battery 851, the DC-DCconverter 852, a converter 853, switches SW1 to SW3, and the displayportion 802 are illustrated in FIG. 10C, and the battery 851, the DC-DCconverter 852, the converter 853, and the switches SW1 to SW3 correspondto the charge-discharge control circuit 850 in FIG. 10B.

First, an example of the operation in the case where power is generatedby the solar cell 811 using external light is described. The voltage ofpower generated by the solar cell is raised or lowered by the DC-DCconverter 852 so that the power has a voltage for charging the battery851. Then, when the power from the solar cell 811 is used for theoperation of the display portion 802, the switch SW1 is turned on andthe voltage of the power is raised or lowered by the converter 853 so asto be a voltage needed for the display portion 802. In addition, whendisplay on the display portion 802 is not performed, the switch SW1 isturned off and the switch SW2 is turned on so that the battery 851 maybe charged.

Note that the solar cell 811 is described as an example of a powergeneration means; however, without limitation thereon, the battery 851may be charged using another power generation means such as apiezoelectric element or a thermoelectric conversion element (Peltierelement). For example, the battery 851 may be charged with a non-contactpower transmission module which is capable of charging by transmittingand receiving power by wireless (without contact), or another chargingmeans may be used in combination.

It is needless to say that one embodiment of the present invention isnot limited to the electrical appliance illustrated in FIGS. 10A to 10Cas long as the nonaqueous secondary battery described in any of theabove embodiments is included.

Embodiment 6

Further, an example of the moving object which is an example of theelectrical appliance will be described with reference to FIGS. 11A and11B.

The nonaqueous secondary battery described in any of the aboveembodiments can be used as a control battery. The control battery can beexternally charged by electric power supply using a plug-in technique orcontactless power feeding. Note that in the case where the moving objectis an electric railway vehicle, the electric railway vehicle can becharged by electric power supply from an overhead cable or a conductorrail.

FIGS. 11A and 11B illustrate an example of an electric vehicle. Anelectric vehicle 860 is equipped with a battery 861. The output of theelectric power of the battery 861 is adjusted by a control circuit 862and the electric power is supplied to a driving device 863. The controlcircuit 862 is controlled by a processing unit 864 including a ROM, aRAM, a CPU, or the like which is not illustrated.

The driving device 863 includes a DC motor or an AC motor either aloneor in combination with an internal-combustion engine. The processingunit 864 outputs a control signal to the control circuit 862 based oninput data such as data of operation (e.g., acceleration, deceleration,or stop) by a driver or data during driving (e.g., data on an upgrade ora downgrade, or data on a load on a driving wheel) of the electricvehicle 860. The control circuit 862 adjusts the electric energysupplied from the battery 861 in accordance with the control signal ofthe processing unit 864 to control the output of the driving device 863.In the case where the AC motor is mounted, although not illustrated, aninverter which converts direct current into alternate current is alsoincorporated.

The battery 861 can be charged by external electric power supply using aplug-in technique. For example, the battery 861 is charged through apower plug from a commercial power supply. The battery 861 can becharged by converting external power into DC constant voltage having apredetermined voltage level through a converter such as an AC-DCconverter. Providing the nonaqueous secondary battery of one embodimentof the present invention as the battery 861 can contribute to anincrease in the capacity of the battery, so that convenience can beimproved. When the battery 861 itself can be more compact and morelightweight as a result of improved characteristics of the battery 861,the vehicle can be lightweight and fuel efficiency can be increased.

It is needless to say that one embodiment of the present invention isnot limited to the electronic device described above as long as theelectronic device includes the nonaqueous secondary battery of oneembodiment of the present invention.

This embodiment can be implemented combining with another embodiment asappropriate.

Example 1

The present invention will be specifically described below withexamples. This example shows results of formation of a positiveelectrode by the method described in Embodiment 2. Note that the presentinvention is not limited to the examples described below.

Charge-discharge characteristics are compared between a cell including apositive electrode with a conductive additive using a graphene oxide (inwhich the atomic ratio of oxygen to carbon (also referred to as O/C. orthe degree of oxidation) was set to 0.547) as a raw material and cellsincluding positive electrodes using a graphene and a reduced grapheneoxide (RGO) whose degree of oxidation is considered extremely low asconductive additives. Charge-discharge characteristics of a cellincluding a positive electrode using conventional acetylene black (AB)as a conductive additive is also compared.

(Fabrication of Positive Electrode Including Conductive Additive UsingGraphene Oxide as Raw Material)

A positive electrode was fabricated using the graphene oxide in whichthe O/C was 0.547. The positive electrode was fabricated in such a waythat positive electrode active material (lithium iron phosphate(LiFePO₄)) particles, a binding agent (polyvinylidene fluoride (PVDF)produced by Kureha Corporation), and the graphene oxide as a conductiveadditive were mixed to form a positive electrode paste and the positiveelectrode paste was applied on a current collector (aluminum) and thenwas dried and reduced. In the fabrication, the compounding ratio(LiFePO₄:conductive additive (graphene oxide):PVDF) in the positiveelectrode paste was set to 93:2:5 (unit: wt %).

(Fabrication of Positive Electrode Using RGO as Conductive Additive)

The reduced graphene oxide (RGO) in this specification means a grapheneformed by reduction of a graphene oxide in advance and is alreadyreduced when dispersed into a dispersion medium. Therefore functionalgroups such as an epoxy group are probably almost eliminated by thereduction reaction. The graphene oxide prepared by the method describedin Embodiment 1 was reduced by heat treatment in which, after held in avacuum for 1 hour, the graphene oxide was increased in temperature to170° C. and held for 10 hours, so that the RGO was formed. The reductionprobably decreases functional groups such as an epoxy group on a surfaceof the RGO to about 10 wt % (weight percent). This RGO was mixed intoNMP, and lithium iron phosphate and PVDF were added thereto, so that thepositive electrode paste was formed. The positive electrode pasteapplied on the current collector was heated and the dispersion mediumwas volatilized; consequently, the positive electrode having thepositive electrode active material layer on the current collector wasfabricated. The compounding ratio (LiFePO₄:conductive additive(RGO):PVDF) in the positive electrode active material layer was set to94:1:5.

(Fabrication of Positive Electrodes Using Graphene as ConductiveAdditive)

As the graphene, a product of Graphene Supermarket was used. Thegraphene had a specific surface area of 600 m²/g, a flake size of about10 μm, and a thickness less than or equal to 1 nm, in which the O/C was0.02. As in the above RGO, the number of bonded functional groups isextremely smaller in the graphene than in the graphene oxide. Thisgraphene is heated at 170° C. for 10 hours by the same method as aboveto form the positive electrodes. The following two positive electrodeswere fabricated: the positive electrode in which the compounding ratioin the active material layer (LiFePO₄:conductive additive(graphene):PVDF) was 94:1:5 and the positive electrode in which thecompounding ratio in the active material layer was 90:5:5.

(Fabrication of Positive Electrode Using Acetylene Black as ConductiveAdditive)

As acetylene black (AB), a powdery product of Denki Kagaku KogyoKabushiki Kaisha was used. The specific surface area was 68 m²/g and theaverage particle diameter was 35 nm. The compounding ratio(LiFePO₄:conductive additive (AB):PVDF) in the positive electrode activematerial layer was set to 80:15:5.

(Measurements of Electrode Conductivities)

The conductivities of the positive electrode active material layersusing the graphene oxide, the graphene included at 1%, the grapheneincluded at 5%, and acetylene black were measured. The measurements gaveresults shown in the following Table 1.

TABLE 1 Positive electrode active Thickness Density Conductivitymaterial layers (μm) (g/cm³) (S/cm) Including conductive additive 30 2.61.3 × 10⁻⁶ using graphene oxide Including conductive additive 48 1.6Measuring using graphene included at 1% limit Including conductiveadditive 43 1.5 5.6 × 10⁻³ using graphene included at 5% Includingconductive additive 23 1.4 1.4 × 10⁻³ using acetylene black (AB)

The conductivity of the positive electrode active material layerincluding the conductive additive using the graphene oxide was thelowest: 1.3×10, S/cm. The conductivities of the positive electrodeactive material layers using the graphene and acetylene black are higherby two or more orders of magnitude.

(Charge-Discharge Characteristics)

The above-described positive electrodes using the graphene formed byreduction performed after the paste including the graphene oxide wasapplied on the current collector, the RGO, the graphene, and acetyleneblack (AB) as conductive additives were included in half cells, andcharge-discharge characteristics of the cells were measured. Here, forconvenience, the cell using the graphene oxide in accordance with thepresent invention as a raw material of the conductive additive isreferred to as a cell D, the cell using the RGO is referred to as a cellE, the cell using the graphene included at 1% is referred to as a cellF, the cell using the graphene included at 5% is referred to as a cell Qand the cell using AB is referred to as a cell H. In the measurements,the charge rate was set to 0.2 C (0.16 C for the cell of the positiveelectrode using AB (cell H)) and the discharge rate was set to 1 C (0.82C for the cell H).

As a result of the measurements, the cell E using the RGO as theconductive additive and the cell F using the graphene included at 1% asthe conductive additive were not able to be charged or discharged atall.

In contrast, battery properties of the cell G using the grapheneincluded at 5% as the conductive additive and the cell H usingconventional acetylene black as the conductive additive were confirmed.Charge-discharge characteristics of the cells G and H in addition tothose of the cell D using the graphene oxide in accordance with thepresent invention as a raw material of the conductive additive are shownin FIG. 12.

FIG. 12 shows charge-discharge characteristics, in which the horizontalaxis represents discharge capacity (mAh/g) and the vertical axisrepresents voltage (V). The heavy line is a curve showingcharge-discharge characteristics of the cell D using the graphene oxideas a raw material of the conductive additive. The thin line is a curveshowing charge-discharge characteristics of the cell G using thegraphene included at 5% as the conductive additive. The dashed line is acurve showing charge-discharge characteristics of the cell H usingacetylene black as the conductive additive.

A discharge curve 901 a and a charge curve 901 b show that the cell Dexhibited good charge-discharge characteristics.

In contrast, the cell G using the graphene as the conductive additivewas found to have low discharge capacity, showing narrowcharge-discharge regions that were plateaus in a discharge curve 902 aand a charge curve 902 b.

Further, the cell H using acetylene black as the conductive additive wasfound to have low discharge capacity, showing no discharge regions thatwere plateaus in a discharge curve 903 a and a charge curve 903 b.

As described above, the cells did not have good charge-dischargecharacteristics with the positive electrodes using the RGO and thegraphene which had almost no functional group as the conductiveadditive, in contrast, the cell had good charge-dischargecharacteristics with the positive electrode formed in such a way thatthe graphene oxide having functional groups bonded by oxidation reactionwas dispersed in the dispersion medium. This may mean that, in thepositive electrode active material layer including the graphene formedby reduction performed after the graphene oxide is dispersed in thepositive electrode paste, the graphene forms a network with high,electron conductivity. On the other hand, in the positive electrodeactive material layer formed by dispersion of the RGO or the graphenehaving almost no functional group in the positive electrode paste, anetwork for electron conductivity is probably not sufficiently formed.Thus, the use of the graphene oxide having a functional group as a rawmaterial of the conductive additive is important in achieving highelectron conductivity of the positive electrode active material layer.

Example 2

Next, experiments were conducted to examine the effect of difference inthe degree of oxidation of the graphene oxide (the number of functionalgroups having oxygen such as an epoxy group) on the charge-dischargecharacteristics of a secondary battery.

(Fabrication of Positive Electrodes)

First, to examine the effect of difference in the degree of oxidation ofthe graphene oxide used on charge-discharge characteristics of asecondary battery, three positive electrodes, a sample A, a sample B,and a sample C, using graphene oxides with different degrees ofoxidation were prepared.

In this example, since graphene oxides with different degrees ofoxidation were necessary to examine the effect of difference in thedegree of oxidation of the graphene oxide on charge-dischargecharacteristics of a secondary battery, graphenes having almost nofunctional group were used as a raw material without use of the graphitepowder described in Embodiment 1. Such graphenes can be oxidized to fromgraphene oxides with oxidizers whose weights are made different whilethe weights of the graphenes are uniform. Thus, graphene oxides withdifferent degrees of oxidation can be fabricated.

For the sample A, the sample B, and the sample C, graphenes produced byCheap Tubes, Inc. were used. The graphenes each had a thickness of 3 nmon average. In the sample A, the sample B, and the sample C, the weightsof the graphenes were each set to 0.25 g. The graphenes were oxidized bybeing mixed into 46 ml of sulfuric acid to which 1.5 g of potassiumpermanganate (KMnO₄), 0.5 g of the same oxidizer, and 0.2 g of the sameoxidizer were added as an oxidizer for the sample A, the sample B, andsample C respectively. The oxidation treatment was performed by stirringat room temperature for 2.5 hours. After that, pure water was added tothe mixture, the mixture was stirred for 15 minutes while being heated,and a hydrogen peroxide solution was added thereto, so that ayellow-brown suspension including a graphite oxide was obtained.

The degrees of oxidation of the prepared graphene oxides for the samplesA to C were measured by X-ray photoelectron spectroscopy (XPS). In themeasurements, monochromatic light Al (1486.6 eV) was used as an X-raysource, the measurement area was set to 100 μm diameter, and theextraction angle was set to 45°. The measurement results were shown inTable 2 and Table 3.

TABLE 2 Sample C O N S O/C Sample A 66.7 32.5 — 0.8 0.487 Sample B 70.728.6 — 0.7 0.405 Sample C 75.3 23.4 0.8 0.4 0.311 unit: atomic %

TABLE 3 Sample C═C C—C, C—H C—O C═O O═C—O Sample A 0.0 25.0 32.0 7.1 2.7Sample B 0.0 30.4 33.1 4.7 2.5 Sample C 0.0 40.6 27.5 4.0 3.2 unit:atomic %

Table 2 shows the quantification values (unit: atomic %) of the elementsC, O N, and S in the samples A to C and the atomic ratio of oxygen tocarbon (also referred to as O/C, or the degree of oxidation). In thegraphene oxide using 1.5 g of the oxidizer for the sample A, the atomicof oxygen is higher than those in the other samples and the O/C is0.487. In the graphene oxide using 0.5 g of the oxidizer for the sampleB, the O/C is 0.405 and in the graphene oxide using 0.2 g of the sampleC, the O/C is 0.311. Thus, by adjustment of the weights of the oxidizersused for oxidation of the graphenes, the graphene oxides with differentdegrees of oxidation were able to be prepared.

Table 3 shows bonding states on surfaces in the graphene oxides of theabove sample A to C listed by state. As the O/C is higher, theproportions of C—C, C—H, and O═C—O are lower while the proportion of C—Ois higher.

Next, using the graphene oxides prepared under the above conditions, thepositive electrodes of the samples A to C were formed. The positiveelectrodes were each formed in such a way that positive electrode activematerial (lithium iron phosphate (LiFePO₄)) particles, a binding agent(polyvinylidene fluoride (PVDF) produced by Kureha Corporation), and oneof the graphene oxides, which were prepared under the above conditions,as a conductive additive were mixed to form a positive electrode pasteand the positive electrode paste was applied on a current collector(aluminum) and then was dried and reduced.

A method of forming lithium iron phosphate used as each of the activematerials of the samples A to Cis described. Lithium carbonate (Li₂CO₃),iron oxalate (FeC₂O₄.2H₂), and ammonium dihydrogen phosphate (NH₄H₂PO₄),which were raw materials, were weighed out such that the weight ratiotherebetween was 1:2:2, and were ground and mixed with a wet ball mill(the ball diameter was 3 mm and acetone was used as a solvent) at 300rpm for 2 hours. After drying, pre-baking was performed at 350° C. for10 hours under a nitrogen atmosphere.

Next, grinding and mixing were performed with a wet ball mill (the balldiameter was 3 mm) at 300 rpm for 2 hours. Then, baking was performed at600° C. for 10 hours under a nitrogen atmosphere.

Next, NMP (produced by Tokyo Chemical Industry Co., Ltd.), which is apolar solvent, was prepared as a dispersion medium. After the grapheneoxide was dispersed into NMP, lithium iron phosphate was added and themixture was kneaded. After PVDF was added to the mixture of the grapheneoxide and lithium iron phosphate as the binding agent, NMP was furtheradded as the dispersion medium and mixed, whereby the positive electrodepaste was formed.

The positive electrode paste formed by the above-described method wasapplied on a 20-μm-thick aluminum foil which is to form the currentcollector, dried in an air atmosphere at 80° C. for 40 minutes, and thendried under a reduced-pressure atmosphere at 170° C. for 10 hours; thusthe graphene oxide in the positive electrode paste was reduced to formthe graphene. The compounding ratio in the positive electrode paste wasset such that the ratio of lithium iron phosphate to the graphene oxideand PVDF was 93:2:5. This compounding ratio was changed by reductiontreatment of the graphene oxide such that the ratio of lithium ironphosphate to the graphene and PVDF was substantially 94:1:5 when thepositive electrode active material layer was formed. However, such asmall change in compounding ratio hardly affects estimation of thedischarge capacity of the secondary battery. Note that, in each of thesamples A to C, anchor coating was performed on a surface of the currentcollector in order to eliminate the influence of interfacial resistancebetween the current collector and the positive electrode active materiallayer.

As described above, the positive electrodes using the three grapheneoxides that differed in the degree of oxidation as raw materials of theconductive additives were fabricated as the samples A to C.

(Charge-Discharge Characteristics)

The positive electrodes of the samples A to C which were fabricated asabove were included in half cells, and charge-discharge characteristicsof the cells (referred to as a cell A, a cell B, and a cell C) weremeasured. When the characteristics were estimated, each cell was in theform of a coin-type cell of a CR2032 type (20 mm in diameter and 3.2 mmhigh). Lithium foil was used as a negative electrode and a 25-μm-thickpolypropylene (PP) film was used as a separator. An electrolyte solutionto be used was formed in such a manner that lithium hexafluorophosphate(LiPF₆) was dissolved at a concentration of 1 mol/L in a solution inwhich ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed ata volume ratio of 1:1. In charging, CCCV at 0.2 C was employed and theupper limit voltage was set to 4.3 V. In discharging. CC at all therates, 0.2 C, 1 C, 2 C, 5 C, and 10 C, was employed and the lower limitvoltage was set to 2′V.

Measurement results of the charge-discharge characteristics of the cellsA and B are shown in FIGS. 13A and 13B. FIG. 13A shows the measurementresults of the charge-discharge characteristics of the cell A includingthe positive electrode of the sample. A using the graphene formed withthe graphene oxide in which the O/C is 0.487 as a raw material. FIG. 13Bshows the measurement results of the charge-discharge characteristics ofthe cell B including the positive electrode of the sample B using thegraphene formed with the graphene oxide in which the O/C is 0.405 as araw material. In each figure, the horizontal axis represents dischargecapacity per active material weight (unit: mAh/g) and the vertical axisrepresents voltage (unit: volt).

As shown in FIG. 13A, the cell A exhibits good battery properties.

As shown in FIG. 13B, the cell B also exhibits good battery properties.

In contrast, the cell C using the positive electrode of the sample Cincluding the graphene formed using the graphene oxide in which the O/Cwas 0.311 did not operate as a battery at all.

As described above, the cells A and B each including the positiveelectrode including the conductive additive using the graphene oxide inwhich the O/C, i.e., the degree of oxidation, was greater than or equalto 0.405 as a raw material were able to exhibit sufficientcharge-discharge characteristics. In contrast, the cell C including thepositive electrode including the conductive additive using the grapheneoxide in which the O/C was 0.3.11 as a raw material was not able toexhibit battery properties. Thus, in the case where the graphene oxidein which the O/C was at least greater than or equal to 0.405 is used,functional groups having oxygen bonded to the graphene oxide aresufficiently included, and accordingly the graphene oxide in thedispersion medium is uniformly dispersed. For this reason, the graphenesformed by the reduction treatment of the graphene oxide performed byheating the positive electrode paste are mixed with high dispersibilityin the positive electrode active material and in surface contact witheach other; consequently, the graphenes form a network with highelectron conductivity, thereby providing battery properties.

In contrast, in the case where the positive electrode including theconductive additive using the graphene oxide in which the O/C is lessthan or equal to 0.311 as a raw material is used, the dispersibility ofthe graphene oxide in the positive electrode paste is low. Therefore thegraphene formed by reduction was not sufficiently dispersed in thepositive electrode active material or was aggregated, which failed toform a sufficient network for electron conduction. This was probably whybattery properties were not able to be obtained.

Example 3

To visually confirm that the use of a graphene oxide having functionalgroups increases the dispersibility in the positive electrode activematerial, scanning electron microscope (SEM) observation was performedon the positive electrode active material layer formed using thegraphene oxide as a raw material of the conductive additive. Forcomparison, a positive electrode active material layer using a RO as aconductive additive and a positive electrode active material layer usinga graphene as a conductive additive were also subjected to SEMobservation.

FIG. 14A shows a SEM image of a surface of the positive electrode activematerial layer formed using the graphene oxide as a raw material of aconductive additive. In the image, the reduced graphene is present notonly in a deep-color portion but also over the entire region. Thegraphene is observed to adhere in a patchy pattern. FIG. 14B is amagnified image of part of FIG. 14A. The plurality of positive electrodeactive material particles 1001 is observed. The positive electrodeactive material particles 1001 are aggregated in batches of several orseveral tens of pieces. Further, in FIG. 14B, as indicated in thedashed-line circle, for example, the deep-color portion represents agraphene 1002. FIG. 15 is a magnified SEM image of part of FIG. 14B. Theimage reveals that the graphenes 1002 spread in such a way as to coat aplurality of positive electrode active material particles 1001 which isaggregated. Since the graphene 1002 is thin, it makes surface contactwith the positive electrode active material particles in such a way asto wrap them along surfaces of the positive electrode active materialparticles. Part of the graphenes 1002 which is not in contact with thepositive electrode active material particles 1001 is stretched, warped,or crimped. In addition, the graphene 1002 is present not only on asurface of the active material layer but also inside the active materiallayer.

FIGS. 16A and 16B, FIGS. 20A and 20B, FIG. 21, and FIG. 22 are SEMimages showing cross sections of the positive electrode active materiallayers formed using the graphene oxide as a raw material of a conductiveadditive.

FIGS. 16A and 16B and FIG. 21 show positive electrode material layersfabricated so that the ratio of lithium iron phosphate (LiFePO₄) to theconductive additive (graphene oxide) and PVDF was 93:2:5 (unit: wt %).In the positive electrode material layer shown in FIGS. 16A and 16B,PVDF (1100) produced by Kureha Corporation was used. In the positiveelectrode material layer in FIG. 21, PVDF (9100) produced by KurehaCorporation was used. In addition, FIG. 21 is a voltage contrast imagewhich clearly shows the graphene oxide.

In the SEM images in FIG. 16A and FIG. 21, the plurality of positiveelectrode active material particles is seen. In part of the images,aggregated positive electrode active material particles can also beseen. Here, white thread- or string-like portions correspond tographenes. Note that among the graphenes, a multilayer grapheneincluding fewer layers may fail to be observed in the SEM images.Further, even graphenes observed far away from each other may beconnected through a multilayer graphenes including fewer layers whichfails to be observed by SEM. The graphenes can be seen like a thread ora string in a gap (void) between the plurality of positive electrodeactive material particles and also adheres to the surfaces of thepositive electrode active material particles. In FIG. 16B, some of thegraphenes in the SEM image in FIG. 16A are highlighted by heavy lines.In both FIG. 16B and FIG. 21, the graphenes 1002 are found to bethree-dimensionally dispersed in the positive electrode active materialparticles in such a way as to wrap the positive electrode activematerial particles 1001. The graphenes 1002 make surface contact withthe plurality of positive electrode active material particles 1001 whilebeing in surface contact with each other. Thus, in the positiveelectrode active material layer, the graphenes are connected to eachother and forms a network for electron conduction.

FIGS. 20A and 20B show a positive electrode material layer fabricated sothat the ratio of lithium iron phosphate to the graphene oxide and PVDFwas 94:1:5 (unit: wt %). In the SEM images in FIGS. 20A and 20B, theplurality of positive electrode active material particles is seen. Inpart of the images, aggregated positive electrode active materialparticles can also be seen. As in FIGS. 16A and 16B and FIG. 21, thegraphenes can be seen like a thread or a string in a gap (void) betweenthe plurality of positive electrode active material particles and alsoadheres to the surfaces of the positive electrode active materialparticles. In FIG. 20B, some of the graphenes in the SEM image in FIG.20A are highlighted by heavy lines. Also in FIGS. 20A and 208, thegraphenes 1002 are found to be three-dimensionally dispersed in thepositive electrode active material particles in such a way as to wrapthe positive electrode active material particles 1001.

FIG. 22 shows a positive electrode material layer fabricated so that theratio of lithium iron phosphate to the graphene oxide and PVDF was94.4:0.6:5 (unit: wt %). In FIG. 22, some of the graphenes in the SEMimage are highlighted by heavy lines. As in FIGS. 16A and 16B, FIGS. 20Aand 20B, and FIG. 21, the graphenes 1002 are found to bethree-dimensionally dispersed in the positive electrode active materialparticles in such a way as to wrap the positive electrode activematerial particles 1001. Further, the graphenes 1002 make surfacecontact with the positive electrode active material particles 1001 whilebeing in surface contact with each other. Even when the graphene oxideis included at 0.6 wt %, in the positive electrode material layer, thegraphenes are connected to each other and form a network for electronconduction.

Thus, regardless of the kind of PVDF or the proportion of the grapheneoxide, the graphene oxide in the positive electrode material layer aresimilarly dispersed three-dimensionally so that a network for electronconduction can be formed.

FIGS. 17A and 17B show SEM observation results of the surface of thepositive electrode active material layer using the RGO as a conductiveadditive. In FIG. 17A, the RGO is present at the deep-color portionwhich is slightly below the center of the figure. FIG. 17B shows amagnified SEM image of this RGO. Although the positive electrode activematerial particles 1003 and the RGO 1004 are in contact with each other,the RGO is seen only around the center of the image and cannot be foundto be in the other region in FIG. 17B. In other words, the RGO has lowdispersibility and is aggregated on the surface of the positiveelectrode active material layer.

FIGS. 18A and 18B show SEM observation results of the surface of thepositive electrode active material layer using the graphene as aconductive additive. In FIG. 18A, several deep-color points correspondto the graphenes. FIG. 0.18B is a magnified image of part of FIG. 18A.Several graphenes 1006 are scattered throughout a plurality of positiveelectrode active material particles 1005. Like the RGO, the grapheneshave low dispersibility and are aggregated.

The above-described results reveal that, when the graphene oxide is usedas a raw material of a conductive additive, dispersibility in a polarsolvent is high because of the functional groups of the graphene oxide,which enables the graphene formed by reduction to be highly dispersed inthe positive electrode active material layer. This demonstrates that thegraphene can form a network for electron conduction in the positiveelectrode active material layer, whereby a positive electrode with highelectron conductivity can be formed.

Example 4

Next, XPS analysis was performed to check the compositions of positiveelectrode active material layers of fabricated positive electrodes inaccordance with the present invention, each of which includes thegraphene formed by reduction performed after a paste including thegraphene oxide was applied on a current collector.

The analysis was conducted for four positive electrodes (a positiveelectrode GN1, a positive electrode GN2, a positive electrode GN3, and apositive electrode GN4) formed by subjecting the positive electrodepastes, in which the compounding ratio of lithium iron phosphate to thegraphene oxide and PVDF was 93:2:5, to treatments under the fourdifferent conditions described below.

(Positive Electrode GN1)

The positive electrode GN1 is an electrode formed by performing noreduction treatment on the graphene oxide and washed after beingimmersed in an electrolyte solution. The electrolyte solution used forthe immersion was formed in such a way that lithium hexafluorophosphate(LiPF₆) was dissolved at a concentration of 1 mol/L in a solution inwhich ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed ata volume ratio of 1:1. The washing for removing a lithium salt wasperformed with DEC. Note that, in the formation of the positiveelectrode GN1, drying for volatilizing the dispersion medium wasperformed but it was treatment at 80° C. for 40 minutes in an airatmosphere and these are not conditions that allow the graphene oxide tobe reduced.

(Positive Electrode GN2)

The positive electrode GN2 is an electrode formed by subjecting thegraphene oxide to electrochemical reduction treatment and washed withDEC like the positive electrode GN1. For the electrochemical reductiontreatment, a coin cell using lithium as a counter electrode wasprepared. The graphene oxide was reduced as follows: discharging wasperformed at 1 C until the reduction potential reached 2.0 V (vs.Li/Li⁺) and the potential was held at 2.0 V for 10 hours.

(Positive Electrode GN3)

For the positive electrode GN3, after the same electrochemical reductionas that performed on the positive electrode GN2 was performed, chargingwas performed at a current of 0.2 C until the potential reached 4.3 Vand the potential was held at 4.3 V until the current value became 0.01C. In addition, the positive electrode GN3 was extracted from the celland washed with DEC after being charged.

(Positive Electrode GN4)

The positive electrode GN4 is an electrode formed by subjecting thegraphene oxide to heat reduction treatment. The heat reduction treatmentwas performed at 170° C. for 10 hours in a reduced-pressure atmosphere.After that, the electrode was washed after being immersed in anelectrolyte solution, like the above positive electrodes.

(XPS Analysis Results)

Table 4 and Table 5 show XPS analysis results of the positive electrodeactive material layers of the positive electrodes GN1 to GN4.

TABLE 4 Positive C—C, O═C—O CF2, electrode C═C C—H C—O C═O CF metal-CO3Positive 0.0 22.8 17.1 3.2 0.6 4.6 electrode GN1 Positive 24.1 18.8 8.41.7 1.9 4.5 electrode GN2 Positive 27.5 15.2 8.8 3.0 2.2 3.0 electrodeGN3 Positive 24.6 19.5 7.7 1.9 1.8 3.9 electrode GN4 unit: atomic %

TABLE 5 Positive C—C, O═C—O, CF2, electrode C—C C—H C—O C═O CF metal-CO3Positive 0.0 47.1 35.5 6.6 1.2 9.5 electrode GN1 Positive 40.5 31.7 14.22.8 3.2 7.6 electrode GN 2 Positive 46.0 25.5 14.8 5.1 3.7 5.0 electrodeGN3 Positive 41.4 32.9 12.9 3.3 3.0 6.6 electrode GN4 unit: atomic %

Bonding states of carbon included in the positive electrode activematerial layers of the positive electrodes GN1 to GN4 were analyzed bywaveform separation of a C1s spectrum and listed by state in Table 4 andTable 5. Table 4 shows the proportions of the bonding states of carbonmeasured by XPS analysis. Table 5 shows the proportions of the bondingstates in all the bonding states.

As shown in Table 4 and Table 5, while the C═C bond was not measured inthe positive electrode GN1 which was not subjected to reductiontreatment, the positive electrodes GN2 to GN4 which were subjected toreduction treatment include the C═C bond at 24.1 atomic % (40.5% of allthe states), 27.5 atomic % (46.0%), and 24.6 atomic % (41.4%),respectively. Further, while the positive electrode GN1 which was notsubjected to reduction treatment includes many C—O bonds (17.1 atomic%), the positive electrodes GN2 to GN4 which were subjected to reductiontreatment include fewer C—O bonds (8.4 atomic %, 8.8 atomic %, and 7.7atomic %, respectively). Although the positive electrode active materiallayers of the analyzed electrodes also include a binder, such reductiontreatment does not change the composition of the binder. Thus, reductiontreatment reduces the number of functional groups bonded to the grapheneoxide and accordingly the number of C—O bonds decreases while the numberof C═C bonds increases.

As described above, reduction treatment after application of a positiveelectrode paste enabled the graphene oxide included in the positiveelectrode paste to be reduced.

The bonding states of carbon included in a positive electrode activematerial layer in a positive electrode formed by such reductiontreatment are as follows: the proportion of the C═C bond is greater thanor equal to 35% and the proportion of the C—C bond is greater than orequal to 5% and less than or equal to 20%; preferably, the proportion ofthe C═C bond is greater than or equal to 40% and the proportion of theC—O bond is greater than or equal to 10% and less than or equal to 15%.

By using such a positive electrode active material layer, a positiveelectrode for a nonaqueous secondary battery which can achieve highelectron conductivity can be provided with a small amount of aconductive additive. A high-density positive electrode for a nonaqueoussecondary battery which is highly filled can be provided with a smallamount of a conductive additive.

Note that the above XPS analysis was performed with the positiveelectrode active material layers including a binder. For comparison,results of XPS analysis performed before and after heat reduction of asimple substance of graphene oxide in the form of powder are shown inTable 6 and Table 7. Table 6 shows the weight proportions of the bondingstates of carbon included the graphene oxide in the form of powder andTable 7 shows proportions of the bonding states in all the bondingstates.

TABLE 6 Positive C—C, O═C—O, electrode C═C C—H C—O C═O CF CF2 Graphene0.0 26.6 25.8 10.0 3.7 0.0 oxide After heat 40.5 20.5 7.1 2.9 1.7 0.0reduction unit: atomic %

TABLE 7 Positive C—C, O═C—O, electrode C═C C—H C—O C═O CF CF2 Graphene0.0 40.3 39.0 15.2 5.6 0.0 oxide After heat 55.7 28.1 9.8 4.0 2.3 0.0reduction unit: atomic %

It can be found that the heat reduction increases the number of the CCbonds while reducing the number of the C—O bonds.

REFERENCE NUMERALS

100: NMP, 101: graphene or RGO, 102: graphene oxide, 200: positiveelectrode, 201: positive electrode current collector, 202: positiveelectrode active material layer, 203: positive electrode active materialparticle. 204: graphene. 300: secondary battery, 301: positive electrodecan, 302: negative electrode can, 303: gasket, 304: positive electrode,305: positive electrode current collector, 306: positive electrodeactive material layer, 307: negative electrode, 308: negative electrodecurrent collector, 309: negative electrode active material layer, 310:separator, 401: container, 402: graphene oxide dispersion liquid. 403:formation subject, 404: conductor, 405: container, 406: electrolytesolution. 407: conductor, 408: counter electrode, 500: secondarybattery, 501: positive electrode current collector, 502: positiveelectrode active material layer. 503: positive electrode, 504: negativeelectrode current collector, 505: negative electrode active materiallayer, 506: negative electrode, 507: separator. 508: electrolytesolution, 509: exterior body, 600: secondary battery, 601: positiveelectrode cap, 602: battery can, 603: positive electrode terminal, 604:positive electrode, 605: separator, 606: negative electrode, 607:negative electrode terminal, 608: insulating plate, 609: insulatingplate, 610: gasket (insulating packing), 611: PTC element, 612: safetyvalve mechanism, 700: display device, 701: housing, 702: displayportion, 703: speaker portion, 704: nonaqueous secondary battery, 710:lighting device, 711: housing, 712: light source, 713: nonaqueoussecondary battery, 714: ceiling, 715: wall, 716: floor, 717: window,720: indoor unit, 721: housing, 722: air outlet, 723: nonaqueoussecondary battery, 724: outdoor unit, 730: electricrefrigerator-freezer, 731: housing, 732: door for refrigerator, 733:door for freezer, 734: nonaqueous secondary battery, 800: tabletterminal, 801: housing, 802: display portion, 802 a: display portion,802 b: display portion. 803: switch for switching display modes, 804:power switch, 805: switch for switching to power-saving mode, 907:operation switch, 808 a: region, 808 b: region, 809: operation key, 810:switching button for showing/hiding keyboard, 811: solar cell, 850:charge-discharge control circuit, 851: battery, 852: DCDC converter,853: converter, 860: electric vehicle, 861: battery, 862: controlcircuit, 863: driving device, 864: processing unit, 901 a: dischargecurve of cell D, 901 b: charge curve of cell D, 902 a: discharge curveof cell C 902 b: charge curve of cell G 903 a: discharge curve of cell1, 903 b: charge curve of cell H, 1001: positive electrode activematerial, 1002: graphene, 1003: positive electrode active material,1004: RGO, 1005: positive electrode active material, and 1006: graphene.

This application is based on Japanese Patent Application serial no.2012-089346 filed with the Japan Patent Office on Apr. 10, 2012 andJapanese Patent Application serial no. 2012-125138 filed with the JapanPatent Office on May 31, 2012, the entire contents of which are herebyincorporated by reference.

1. (canceled)
 2. A method for manufacturing a lithium ion secondarybattery, comprising the steps of: immersing a positive electrode, aseparator and a negative electrode in an electrolytic solution; stackinga positive electrode can, the positive electrode, the separator, thenegative electrode and a negative electrode can in this order; andperforming pressure bonding on the positive electrode can and thenegative electrode can with a gasket interposed between the positiveelectrode can and the negative electrode can.